Old rucksack with disintegrating waterproofing
August 9, 2017 7:21 AM   Subscribe

Found a Lowe Alpine II at a thrift store - yay! Upon closer inspection, the interior waterproofing - it seems to be some kind of thin rubberized layer - appears to be rotting (boo). The waterproofing is supposed to be clear, but is now opaque white in places, and is sticky. If I touch it the stickiness transfers to my fingers (gross). Can I somehow save this? Thank you!
posted by carter to Sports, Hobbies, & Recreation (9 answers total)
I think you need to scrub off as much as you can, and then re-apply a new coating. Or just have a non-waterproof rucksack. This guide to tent care might be helpful: A Step-by-Step Guide to Ultimate Tent Care, and the search term DWR (durable water repellent) finish might be useful for searching.

It might or might not work, but if you otherwise like the pack and are willing to spend some time and a few bucks on waterproofing it might be worth it.
posted by mskyle at 7:25 AM on August 9, 2017 [1 favorite]

I'd pick up some acetone and denatured alcohol and see if either acts as a good solvent on it. Neither should affect the nylon pack material. Then lots of rags and solvent. Turn the pack inside out and start wiping.
posted by humboldt32 at 8:06 AM on August 9, 2017 [1 favorite]

Thanks both, the solvent is a good idea. I just contacted Lowe Alpine to see if they can recommend a solvent to do this.
posted by carter at 8:16 AM on August 9, 2017

For what it's worth I've tried a number of waterproofing treatments for clothing and camping gear over the years and none of them will give you better than mild water resistance. Better than nothing, but not that much better. My favorite is still Kiwi Camp Dry, which is a silicone spray that's easy to use and good value. Pair with inexpensive silnylon or PU-coated water resistant (claims to be waterproof but again, not really) stuff sacks if you have gear that really needs to stay dry. You should then be fine against anything short of prolonged immersion.
posted by Anticipation Of A New Lover's Arrival, The at 8:31 AM on August 9, 2017

Also, while acetone should be fine for nylon, I'd be cautious. Zippers, stitching, hardware etc. won't necessarily also be acetone-compatible. I'd be more inclined to go with a mechanical solution like a stiff brush and maybe some hot, soapy water.
posted by Anticipation Of A New Lover's Arrival, The at 8:40 AM on August 9, 2017 [2 favorites]

It's polyurethane. It's factory applied to the fabric before it's made into the bag. It doesn't really make the bag waterproof. There are way too many seams and zippers in a backpack like that to make it waterproof, even brand new. Once water starts to find a way through a seam somewhere, it generally wicks through in other places as well. Sea Line and others make really waterproof bags where the material is PVC (or a replacement) and the seams are welded. No sewing! You see bike couriers, canoeists and kayakers with those. If you want waterproof, that's waterproof. Backpacks like yours are a bit water resistant, but canoeists have largely given up on them because they will soak through totally on a rainy day in a canoe. Nothing beats a bag like yours for hiking and backpacking though.

The polyurethane is there to add a bit of water resistance, and help the polyester or nylon face fabric last longer. I worked at a gear shop, and we did a lot of repairs, but had no procedure to reapply polyurethane. In general that stuff lasts a long time and by the time it starts to peel, there are other seams and zips that are ready to go as well. But I can imagine a situation where the bag was stored in a garage and faced temperature and moisture extremes that would age the polyurethane faster than the zips and seams.

So I think your strategy should be to try to peel off as much of the sticky peeling polyurethane as you can. Avoid solvents unless the manufacturer tips you off to something that will work. There are other layers of plastic, foam, and polyurethane in the suspension of the bag that might not take kindly to it. Turn the bag inside out, and see what you can pull off. The scrubby brush and hot water idea above is a good one. I might try using it just like that for a while. The sun ages polyurethane. If it's sticky, but doesn't come of the fabric, put it inside out again and let it sit in the sun for a bit. Then re-peel.

You can try a polyurethane repair goo like this and see how it works. At the gear shop, we'd see the odd backpack come in for a repair that had had these products applied, and it looked like more trouble than it's worth. In general, silicone doesn't stick to polyurethane, but it does get used on the outside of the fabric where it will stick to nylon or polyester. It makes water bead up, but doesn't physically block holes, which is what you need for waterproofness.
posted by thenormshow at 8:56 AM on August 9, 2017 [7 favorites]

I've also had the interior membrane of a pack start to peel and crumble. I would attempt one of the solvents above. BUT I wouldn't bother trying to re water proof it. If you are going to be hiking in the rain, just get a pack cover. 1000% more effective and you can take it on/off as needed.

For Example:
posted by TomFoolery at 8:58 AM on August 9, 2017

Thanks so far everyone. Yes to clarify I'm very happy just to remove, and not reapply anything. I just want the stuff off!
posted by carter at 9:16 AM on August 9, 2017

For whatever reason I've been turning this over throughout the day, and come to think of it I'm pretty sure I've encountered old bags with similar PU breakdown as yours—only the coating had gone past the sticky stage to super dry and crumbly. I never had your problem, because like thenormshow suggests in his excellent comment above, I just peeled it out. It was easy, because it was so dry.

I have a hunch that sunlight will help your bag's coating transition from sticky to dry. Ultraviolet radiation is the enemy of polyurethane, and I bet that if you left your bag hanging inside out on a line on a hot, sunny day it might dry out and become more peel-able. Might take more than one day, but it costs you nothing to try and would be super easy if it worked.
posted by Anticipation Of A New Lover's Arrival, The at 7:12 PM on August 9, 2017

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