How to Choose Waterproof Bags That Survive Hot Spring Chemistry, Not Just Water

Why hot springs destroy bags by chemistry rather than by volume

Most buyers choose waterproof hot spring bags as though the threat were the quantity of water, and it is not. On a bathing day the bag never goes underwater. It sits in a changing room or on a wooden bench in an atmosphere that is typically 80 to 100 per cent relative humidity at 30 to 45 degrees Celsius, it gets splashed by run-off from a wet swimsuit, and it is packed damp at the end of the day and left closed for the journey home. That is a low-volume, long-duration, chemically aggressive exposure, and it is almost the opposite of the short, high-volume exposure that a submersion rating describes. onsen travel gear therefore has to be specified on a different axis entirely: chemical resistance, vapour resistance and dryability, in that order, with raw water exclusion a distant fourth.

The practical consequence is that the common buying shortcut fails here in a specific and predictable way. A bag selected because it has a welded shell and a high ingress rating will keep rain out perfectly and will still come back from a weekend in a sulphur town with blackened hardware and a coating that has begun to hydrolyse, because the rating says nothing about the atmosphere the bag sat in for eight hours. Meanwhile a simpler bag with acetal hardware, a polyether-based coating and a genuinely sealed wet compartment will come back looking almost new. Specification order matters more than specification level in this category.

• Exposure duration is the variable that changes everything: a changing room holds a bag at high humidity for hours, and coating chemistry is time-and-temperature driven rather than pressure driven.
• The aggressive agents are dissolved, not liquid: hydrogen sulphide vapour, chloride ions, and low pH all attack materials through a thin film of moisture rather than through immersion.
• Damage is cumulative and invisible until it is not: hardware darkens gradually, then seizes; coating loses flexibility gradually, then cracks at a fold.
• The journey home is part of the exposure: a damp bag closed for a four-hour train ride completes the damage that the bathhouse started.
• Drying is the intervention that works, and it is the one travellers skip, because a wet bag gets packed first and unpacked last.

It is worth stating the counter-case honestly, because credibility matters here. If a traveller visits a spring town once, keeps the bag in the hotel room while bathing, and dries it properly that evening, almost any reasonable waterproof bag will survive fine. The specification in this guide is for the repeat pattern: four or more bathing trips a year, multiple springs per trip, a bag that lives in the changing room rather than the bedroom, and a traveller who will not reliably dry it. That is the profile that loses bags, and it is also the profile that buys a second one.

What is actually in the water: sulphur, salt, acid and the pH range that matters

Spring water is not one thing, and the differences matter more than the similarities. The three chemistries that damage bags are sulphide, chloride and low pH, and a given spring may deliver any combination of them. Sulphur springs carry dissolved hydrogen sulphide and, in the vapour above the water, enough sulphide to blacken copper, silver, brass and nickel within hours. Salt springs carry sodium chloride plus magnesium and calcium salts, which drive pitting corrosion in stainless steel and leave crystalline deposits that abrade coatings as they dry. Acidic springs run at a pH as low as 2 in extreme cases and commonly between 3 and 5, and that is low enough to hydrolyse ester-based polyurethane directly.

Two details from that table drive most specification decisions. The first is that hardware and coating fail to different agents, so a single "chemical resistant" claim is meaningless without naming which chemistry. The second is that iron staining is permanent on pale fabric, which is a design argument for darker shells and darker webbing on any product intended for spring towns, independent of any performance consideration.

For a brand putting numbers into a brief, the useful measurement is not a single pH but pH plus total dissolved solids plus sulphide concentration, all of which a local tourism association or a published spring analysis will usually state. Public water chemistry reference material from the United States Geological Survey sets out how these species are measured and reported, and the corrosion testing convention used to rank hardware resistance is published by the International Organization for Standardization. The material-side trade-offs against these chemistries are treated in more depth in our page on chemical resistance of waterproof materials.

Hardware that survives sulphide vapour: what to specify instead of plated metal

If there is one component that visibly dies on a spring trip, it is the hardware, and it dies because most bag hardware is chosen for showroom appearance rather than for chemistry. Zamak, brass and nickel-plated steel all react with sulphide. The reaction is not subtle: copper and silver form black sulphides, brass dezincifies and goes pink then brown then black, and nickel plating that has any porosity at all develops black spots that spread under the plate. A slider that has corroded internally does not merely look bad, it stops running, and a slider that will not run is the end of a waterproof closure.

The recommendation that falls out of that table is unglamorous and correct: specify acetal for every functional part, 316 stainless only where a metal part is genuinely needed, and treat any bright decorative metal as a liability. Acetyl hardware is also quieter and lighter, which matters in a bathing context where a bag is opened in a quiet changing room and carried through a wooden corridor. The selection logic, including the load and slip testing that should accompany it, is set out in our page on custom hardware selection.

• Specify acetal or POM for buckles, sliders, adjusters and D-rings as the default, and ask for the grade by name rather than accepting "plastic hardware".
• Where metal is unavoidable, ask for 316 rather than 304 stainless and confirm it on the sample with a magnet and a mill certificate if the volume justifies it.
• Avoid hidden plated press-studs and rivets, because they are the parts nobody rinses and the first to seize.
• Choose matte or dark finishes for any visible metal so early tarnish is less visible; the corrosion still happens, it just stops being a complaint.
• Make the main slider a replaceable part on any bag sold into spring markets, because it is the single most likely component to fail.

Coatings under warm vapour: why hydrolysis accelerates at 40 to 60 degrees

The second failure mechanism is thermal, and it is the one that surprises people because it happens to a bag that never got obviously wet. Polyurethane coatings degrade by hydrolysis: water molecules attack the ester links in the polymer backbone, the chain shortens, the coating loses flexibility and eventually goes tacky, then cracks, then peels. Hydrolysis is a chemical reaction, so its rate rises with temperature in the way all chemical reactions do. A rough and useful rule of thumb for ester-based polyurethane is that the rate doubles for roughly every ten degrees Celsius rise in temperature, which means a bag sitting in a bathhouse changing room at 40 degrees is ageing several times faster than the same bag in a hotel room at 20.

The material answer is to specify a polyether-based thermoplastic polyurethane rather than a polyester-based one, because the polyether backbone is far more resistant to hydrolysis, and to avoid PVC in warm humid service because plasticiser migration accelerates with heat and leaves the film brittle and odorous. Film thickness matters too: a thicker TPU film has more material to lose before it becomes a functional problem. The thermal ageing behaviour of these materials, and the accelerated tests that predict it, are covered in our page on thermal properties of waterproof materials, and the chemistry differences are set out under coatings explained. Accelerated ageing protocols are described in accelerated ageing tests.

There is also a design answer, and it is cheaper than a material upgrade: give the bag a shape and a construction that can actually be dried. A welded interior with no fabric liner traps less water than a laminated textile one, a light-coloured interior makes residual damp visible, and a bag that opens fully flat dries in an hour where a bag that opens to a narrow mouth dries overnight. Two of the three most common spring-trip complaints are really drying complaints.

Zoning a bathing day: wet swimsuit, damp towel, dry clothes

On a bathing day the bag carries three moisture states at once, and the whole organisational problem is keeping them apart. A worn swimsuit holds a surprising amount of water: a typical one-piece or a pair of swim shorts retains roughly 150 to 400 millilitres after a squeeze, and a bath towel used at an outdoor spring holds considerably more. A dry change of clothes, a phone, a wallet and any electronics must share the same bag as that wet mass, usually for several hours, in a warm vehicle or train. Nothing about that is exotic; it is simply the highest internal moisture load of any travel scenario short of a kayaking trip.

• Give the wet items their own fully welded compartment or pouch, not a mesh pocket in the main space; mesh separates dirt but not water.
• Put the wet compartment at the base or in a side panel so any leak drains away from the main compartment rather than into it.
• Size the wet pouch to the actual load: 8 to 12 litres handles a suit plus a towel for one person, and 15 to 20 litres for two.
• Keep electronics and documents in a separate dry pouch inside the dry zone, so there are two barriers rather than one.
• Pack a second small pouch for the towel only if the towel is genuinely wrung out; a dripping towel belongs in the wet zone even if it feels unfair to the swimsuit.
• Carry one spare dry pouch folded flat; it costs almost nothing and it is the fix for the day you buy something wet or forget to wring the suit.

The separation logic is standard in the wider category and worth reading once rather than re-deriving: the construction options and their real performance are compared in our page on wet and dry separation design. For spring use the specific requirement is that the wet compartment be welded rather than taped, because taped seams in contact with warm chloride water for hours is exactly the condition under which seam tape starts to lift. A dedicated waterproof wet-clothes bag is the low-cost way to add a second barrier to a bag that was not designed for this.

Etiquette as a design constraint: the changing room, not the pool

Bathing etiquette is usually presented to travellers as courtesy, and for bag design it is better understood as a constraint with physical consequences. In a Japanese onsen the bag does not enter the bathing area; it stays in the changing room, in a locker, a basket or on a shelf, and it stays there for the whole session. Clothing and towels go to specific places, the small towel stays out of the water by convention, and the changing room is a quiet shared space where a bag is opened and closed several times in close proximity to other people and to their dry clothing.

• The bag must be small enough for a standard locker or basket, which means external dimensions matter more than litre count; see the sizing section below.
• The closure must be operable quietly and one-handed, because the bag is opened on a lap or a low bench while sitting next to other people.
• Hardware should not rattle or scrape; acetal hardware and a fabric-covered slider pull are quieter than metal on wood or on a metal locker.
• A hanging loop or a shaped handle is genuinely useful, because changing rooms often have hooks and rarely have clean flat floor space.
• Access should be from the top or the front panel, so the bag can be opened while it is standing in a basket rather than laid out on a wet floor.
• Valuables belong in a small pouch that can be taken to the locker rather than left in the bag, since many facilities treat the changing room as semi-public.

Etiquette guidance for visitors is published by the Japan National Tourism Organization, and facility-level rules vary more than travellers expect: some allow a small bag in the changing area, some require lockers, and a minority of traditional bathhouses have no secure storage at all. That variation is itself a specification argument: the bag should be designed so that its valuable contents can be lifted out in one small pouch. Our wider page on waterproof bags for hotels and hospitality covers the related question of how facilities specify guest-loan bags for wet areas.

Tatami, wood and the moisture a wet bag brings indoors

The third environment in a spring trip is the room you sleep in, and in a traditional ryokan it is the least forgiving surface a bag will ever meet. Tatami is woven rush over a rice-straw or board core; it absorbs water, stains, and stays damp for a long time because air does not circulate under a bag placed on it. Unfinished or lightly finished wood behaves similarly at the joints and edges. A bag with a wet base set down on tatami for an evening will leave a mark that the guest is charged for, and the complaint will be attributed to the bag rather than to the traveller.

• Specify a base panel that sheds water rather than one that holds it: a smooth welded TPU base wipes dry, a laminated fabric base stays damp.
• Add a hanging loop or a hook-compatible grab handle so the bag can be hung in the entrance area rather than set down.
• Avoid hard protruding feet or studs on the base, which concentrate pressure and mark soft flooring more than a flat base does.
• Choose darker base and trim colours, because any residual damp on tatami shows as a ring and a dark base hides the transfer.
• Consider a packable mat or a flat dry pouch that can be placed under the bag, which is the cheapest possible protection for a guest room.

Condensation deserves a note here because it is the mechanism that surprises people in heated rooms. A cold bag carried in from a winter outdoor spring into a heated room at 22 degrees will condense moisture on its own outer surface, and that condensate drips onto whatever it is standing on. Wiping the outside of the bag before bringing it into a tatami room is a five-second habit that prevents the most common room-damage complaint. The design version of the same idea is covered in our page on anti-condensation design.

Sizing for the bathing trip: capacity that fits a changing-room basket

Capacity sells and dimensions decide. On a spring trip the binding constraint is not how much the traveller wants to carry, it is the opening of the locker or basket in the changing room. Coin lockers at Japanese bathhouses commonly have an internal width of roughly 250 to 350 millimetres and a depth of 400 to 550 millimetres, while the open baskets used in many traditional facilities are around 400 by 300 millimetres with sides under 200 millimetres high. A 60 litre travel pack will not go into either, and a bag that cannot be stored is a bag that gets left on a bench in the wettest room in the building.

Two design conclusions follow. The first is that a compressible soft construction beats a moulded one for spring travel, because a soft bag can be squeezed into a basket where a framed pack cannot. The second is that splitting the load across two medium bags is usually better than one large bag, both for storage and because it lets the wet items travel separately from the dry ones. Compression and packing discipline for this kind of trip are covered in our guide to compression bags for travel.

A failure and maintenance calendar for spring travellers

Everything in this category improves dramatically with a routine, and the routine is short enough that it is worth stating as a checklist rather than as advice. The critical window is the first thirty minutes after leaving the water, because that is when the dissolved minerals are still mobile and have not yet crystallised inside a zip track or under a coating edge. Rinsing with clean fresh water at that point removes most of the aggressive species before they do any work.

• Within thirty minutes of the last session, rinse the bag and all hardware under clean fresh water, including the zip track and the inside of the buckles.
• Wipe the hardware dry immediately; water left standing on a slider is what carries sulphide and chloride into the mechanism.
• Open every compartment fully and dry at room temperature away from direct heat; do not use a hairdryer, a radiator or a heated floor.
• Rinse the wet compartment separately and leave it open overnight, because it is the part that holds the highest mineral load.
• Once per trip, run the zips through their full travel while wet and then again after drying, to flush grit out of the track.
• At the end of a trip, store the bag open and loosely packed in a dry place; a bag stored closed and slightly damp will smell within days.
• Every three or four trips, inspect slider bodies, rivets and any exposed metal for darkening, and treat the first sign of tarnish as a signal to change hardware specification on the next purchase.

What not to do matters as much as what to do. Do not machine wash a coated bag, do not use solvents, do not store it in direct sun to dry it, and do not seal it into a plastic cover for the journey home. Each of those is a common response to a wet bag and each of them does measurable harm. Care and cleaning limits are summarised across our quality and inspection guidance, and the environmental limits used to validate this kind of routine are described under temperature and humidity testing.

Testing a bag against spring water rather than against clean rain

A traveller can run a meaningful version of the laboratory test at home in an afternoon, and a brand should run a proper version before signing off a spring-oriented product. The principle is that the test fluid and the test temperature must resemble the service environment. Clean water at room temperature tests almost nothing relevant; warm water with a realistic mineral load at 40 degrees tests the mechanism that actually fails. For a traveller the practical version is a soak-and-hang test on a sample or on an old bag before committing the good one.

• Fill a basin with water at 40 degrees and add a mineral load representative of the destination: a few grams of salt per litre for a chloride spring, or a small amount of a sulphide-bearing solution if safe and legal to obtain.
• Immerse a hardware sample or an old bag partially for four hours, then leave it sealed in a warm humid container overnight to mimic the packed-damp journey home.
• Inspect at 24 hours for hardware darkening, coating tackiness at folds, and any seam tape lift.
• Cycle the closure twenty times before and after and note any change in pull force; a slider that has stiffened has already begun to corrode.
• Repeat across three cycles and compare two candidate bags side by side rather than judging one in isolation.
• For a brand, convert the same protocol into a defined test with fixed concentrations, temperature and duration, and require it on every reorder.

The brand version of this belongs in the specification as a named test with acceptance criteria, not as a sentence about chemical resistance. Realistic acceptance criteria for a spring-oriented product are: no visible tarnish on functional hardware after three cycles, no coating tackiness or delamination at fold points, no seam lift, and closure pull force within fifteen per cent of the pre-test value. The wider framework for turning use conditions into test conditions is set out in our page on lab-to-real-world test validation, and the baseline water-resistance measurement is explained in hydrostatic head ratings.

Odour, towel rot and the microbial side of a wet bag

The failure travellers complain about most often is not a leak or a broken buckle, it is the smell, and it arrives later than they expect. A bag packed damp after a bathing session carries warm water, skin cells and textile fibres into a closed, dark, warm space for the length of a journey home. That is close to an ideal microbial growth condition. Within twenty-four to forty-eight hours the wet compartment develops the characteristic sour odour, and within a week the same odour has migrated into the main compartment and into anything textile that was packed near it. A towel left in that environment for two days can mildew outright.

The design response has three parts. The wet compartment should be welded and wipe-clean so it can actually be sanitised rather than merely dried. The interior should be light-coloured so damp and residue are visible. And the bag should be constructed so that it can be opened fully flat, because a bag that cannot be opened fully cannot be dried fully, and a bag that cannot be dried will smell regardless of how good the coating is. Where a brand wants an additional claim for humid markets, an antimicrobial finish can help, but it is an addition to a dryable construction and never a substitute for one; the realistic scope and limits are described in our page on antimicrobial treatments.

• Empty and open the wet compartment the same evening, every time, without exception; this single habit prevents most odour complaints.
• Wring the swimsuit as dry as possible before packing it, since the water it carries is the entire load the compartment has to manage.
• Wipe the wet compartment with a dilute mild detergent at the end of a trip rather than waiting for a smell to appear.
• Never pack a towel and a swimsuit together in a closed pouch for more than a few hours.
• Air the whole bag open overnight once during any trip of three days or more.

What a brand should specify for a hot spring and onsen line

For a brand building a spring or bathing line, the product structure should follow the trip rather than the product category. The three pieces a spring traveller actually buys are a locker-sized main bag of 18 to 28 litres, a welded wet pouch of 8 to 15 litres, and a small valuables pouch of 1 to 3 litres. That ladder covers the whole use case, shares one material story, and lets a programme reach 500 pieces per style across two or three colourways without taking four separate bets.

• Fix the material tier first: a polyether-based TPU film or laminate in the 0.35 to 0.55 millimetre range, with a welded rather than taped wet compartment.
• Specify acetal hardware across the functional parts and reserve any metal for 316 stainless, named on the bill of materials rather than implied.
• Ask for a defined chemical and thermal ageing test with numeric acceptance criteria, not a general chemical-resistance statement.
• Choose darker shells, darker webbing and darker base panels, because iron staining and scale are permanent on pale materials and will generate returns.
• Design the wet pouch as a standalone SKU, since it is the highest repeat-purchase item in the category and it sells to travellers who already own a bag.
• Make the main slider a serviceable part and stock spares, because hardware is the first thing to fail and the cheapest thing to replace.
• Plan packaging and labelling around the care routine: a card that says rinse within thirty minutes reduces complaints more effectively than any material upgrade.

On commercial mechanics, minimum order quantity is 500 pieces per style, which is straightforward to reach across a three-piece ladder in two colourways when the range is planned as a single programme. Colour and trim consistency across reorders matters more here than in most categories because travellers buy the pouch after the bag and expect them to match; the mechanics are set out in our page on minimum order quantities, and the development sequence in lead times for custom orders. Sizing flexibility for locker-specific dimensions is covered under size and dimension customisation. To turn those specifications into actual dates and a quotation, review the process from first enquiry through sampling into bulk production and send the capacity ladder, hardware specification and target volumes. Sampling takes 6–10 working days, bulk production runs 35–50 days, and quotations are issued FOB Xiamen.

A buying checklist by traveller type

Buying to a specification survives model changes, so the checklist below is written as specification rather than as product names. Every line is checkable in a shop in under a minute, and the first three lines are the ones that decide whether the bag comes back from the trip in usable condition.

• Day visitor: 12 to 18 litres, width under 300 millimetres, acetal hardware, one welded wet pouch, opens fully flat, dark base panel.
• Overnight ryokan guest: 18 to 28 litres, compressible soft construction, welded wet compartment at the base, hanging loop, quiet closure.
• Spring-hopping traveller with a car: 25 to 35 litre main bag plus a separate 15 to 20 litre wet bag kept in the vehicle, never packed together.
• Family traveller: two medium bags rather than one large one, at least two wet pouches, and a spare dry pouch folded flat in each.
• Repeat traveller bathing weekly: polyether TPU shell named on the specification, serviceable slider with a spare in the wash kit, and a strict same-evening drying habit.
• Any traveller: reject any bag with bright brass, nickel-plated or zamak hardware, regardless of how good the fabric looks on the shelf.

If a traveller takes only one idea from this page, it should be the reordering of priorities: chemistry first, then hardware, then dryability, and only then the water-exclusion rating that the market sells on. A bag built in that order will survive years of spring travel. A bag built in the market’s order will survive one season and come back with black hardware and a smell.

Frequently Asked Questions

Q1. Do I need a fully submersible bag for a hot spring trip?

No. The bag never goes underwater. What matters is resistance to warm mineral vapour, sulphide and chloride attack on hardware, and the ability to dry out fully. Hydrolysis resistance beats a submersion rating here.

Q2. Why does bag hardware go black at sulphur springs?

Hydrogen sulphide in the vapour reacts with copper, silver, brass and nickel to form dark metal sulphides. Brass also dezincifies and plated parts corrode under the plating. Acetal and 316 stainless are the practical answers.

Q3. Which hardware materials should I look for on a spring bag?

Acetal or POM for buckles, sliders and adjusters, and 316 stainless only where metal is genuinely required. Avoid zamak, brass, bronze and nickel-plated steel, all of which tarnish or pit quickly.

Q4. Does warm vapour really damage a waterproof coating?

Yes. Ester-based polyurethane hydrolyses, and the rate roughly doubles for every ten degree rise in temperature. A bag in a 40 degree changing room ages several times faster than the same bag in a hotel room.

Q5. What coating should I specify for hot spring use?

A polyether-based TPU rather than a polyester-based one, because the polyether backbone resists hydrolysis far better. Avoid PVC in warm humid service, since plasticiser migration accelerates with heat.

Q6. How much water does a wet swimsuit actually carry?

Roughly 150 to 400 millilitres after a squeeze, and a bath towel considerably more. That is the entire internal moisture load the bag has to manage, so a genuinely welded wet compartment is the key feature.

Q7. What size bag fits a Japanese onsen changing room locker?

Check dimensions rather than litres. Coin lockers commonly take a width of 250 to 350 millimetres and a depth of 400 to 550 millimetres, so a 60 litre pack will usually have to sit on a bench instead.

Q8. Can I take my bag into the bathing area?

No. Bags stay in the changing room in a locker or basket. That is why a locker-sized bag with quiet, one-handed closure and a hanging loop is the right specification for this use.

Q9. How do I stop a bag smelling after a bathing trip?

Empty and open the wet compartment the same evening, wring the swimsuit thoroughly before packing, and wipe the compartment with mild detergent at the end of the trip. Odour comes from moisture that was already inside.

Q10. Will acidic spring water damage the fabric itself?

It can. Springs at pH 2 to 5 accelerate coating hydrolysis and dye bleed. Rinsing with clean fresh water within thirty minutes of leaving the water removes most of the aggressive species before they do any work.

Q11. What should I do about iron staining from iron-rich springs?

Prevention only. Iron staining is permanent on pale fabric and webbing, so choose darker shells and darker webbing for spring destinations and rinse promptly. Once stained, the marking will not wash out.

Q12. How should I dry a bag after a hot spring visit?

Open every compartment fully and dry at room temperature away from direct heat. Avoid hairdryers, radiators and heated floors, which degrade coatings and can distort welded seams.

Q13. Is a separate wet bag better than a built-in wet compartment?

Both work, and the safest configuration is both: a welded built-in compartment plus a standalone pouch. A standalone pouch adds a second barrier and is the item spring travellers buy most often as a repeat purchase.

Q14. Can I put a wet bag on tatami in a ryokan?

Not without protection. Tatami absorbs water and stains, and it stays damp because air does not circulate under a bag. Wipe the base dry and use a hanging loop or a flat dry mat underneath.

Q15. How often should a spring bag be replaced?

Judge by hardware and coating condition rather than by years. A bag used monthly at sulphur springs typically needs a slider within one to two years, while a bag with acetal hardware and a polyether shell can last considerably longer with proper drying.

Q16. What should a brand specify first for a hot spring product line?

The material tier and the hardware alloy, before capacity or colour. A polyether TPU shell plus acetal hardware and a welded wet compartment resolves most of the failure modes, and both must be named on the bill of materials.

Q17. How can I test a bag for spring resistance before buying or specifying?

Soak hardware or a sample at 40 degrees in water with a representative mineral load for four hours, seal it warm overnight, and inspect at 24 hours for tarnish, tackiness at folds and seam lift. Repeat three cycles and compare candidates side by side.

People Also Ask

Do hot springs really damage waterproof bags?

Yes, but chemically rather than by volume. Sulphide blackens hardware, chloride pits metal, and warm vapour accelerates coating hydrolysis. Volume ratings do not predict any of it.

What hardware survives sulphur springs?

Acetal or POM for all functional parts, and 316 stainless where metal is required. Zamak, brass, bronze and nickel-plated steel all tarnish or pit quickly.

Why does my bag smell after an onsen trip?

A damp bag packed closed for the journey home is close to an ideal microbial growth condition. Open and dry the wet compartment the same evening.

What size bag fits an onsen locker?

Check width and depth rather than litres. Most coin lockers take 250 to 350 millimetres wide and 400 to 550 millimetres deep, so avoid large framed packs.

How do I protect tatami from a wet bag?

Wipe the base dry, hang the bag where possible, and place a flat dry mat underneath. Tatami absorbs water, stains, and stays damp under a bag.

Which coating resists hot spring water best?

A polyether-based TPU film or laminate. It resists hydrolysis far better than polyester-based polyurethane, and it avoids the plasticiser loss PVC shows in warm humid service.