Not sure I want to know why you have moisture under your mattress... but anyway, who told you to put water seal on the plywood? I don't think you can apply polyurethane over that?

I'd start again with marine plywood, sand, and apply an exterior/marine polyurethane. It still might be difficult to seal the edges.

LONG story about the mattress/bed furniture (bad design ) but the boards I am using for support I am waterproofing them just in case moisture gets under the boards-a big boxed in area that sits on the floor. This old cottage has bad insulation and the air goes from cold at night to warm in the day --BIG swing too. and the floor is like frozen ice (no insulation between under the house crawl space, literally just 2 layers of wood,the carpet pad, and carpet. Anyway---sorry not to get into all that, wrong forum topic---but now I'm trying to waterproof/moistureproof the boards the mattress will sit on and I'm soooo not familiar with the sealers. Was hoping you guys who seal decks might be able to help. My planks are just about dry, and I'm understanding (correct me if I'm wrong) so not to bother with the polyurethane over it? My mattress is coming Tuesday--I literally just ruined a $3k mattress due to the center it becoming a valley on cheap thin veneer boards and moisture making puddles either from condensation thru the cheap boards or down thru sweat via the mattress.or air between the board and mattress.... I don't know :( all I know is I'm
1) putting strong plywood down now
2) waterproofing the wood to make a moisture block in either direction
3) putting a waterproof protective mattress cover on the entire mattress. They actually sell those--and they breathe.I HOPE!
4) hopefully all this above will keep my new bed (another $3k bed Tempur) or I will be on the couch for at least another year. :(

1) I wouldn't call 1/4" plywood "stong".
2) It sounds like the last thing you need is to _block_ the moisture - it sounds like you want to add ventilation. Can't you raise the boxed part above the floor so air can get under it?
3) A waterproof mattress cover that breathes will probably stop _liquid_ water but still pass water vapor.
4) I wouldn't put a $3K mattress on wood that has been treated with water seal. Sealing a deck exposed to sun, rain, and snow is different from creating a water barrier under a bed!

Thanks for the advice Dennis. I really do appreciate it. I don't know what to do. On the "furniture" threads folks are trying to help also. I feel like the ultimate screw up. I bought nice furniture from Pottery Barn with a flaw that others have had the same results but I want to keep the bed, so I'm doing everything I can. I guess I can go to HD and get more wood cut. --the reason why it is 1/4" is because it is replacing the old fake wood that sits in the middle of the platform bed- that had no support. The 1/4"plywood is way stronger than the original thank god. plus I am putting cement cinderblocks under it throughout. this is so frustrating... :

If you can get it all up off the floor by even using 3/4" blocks so that there is an air space without any of it in direct contact. Yor bigger concern should be your crawl space issues as your subflooring is going to deteriorate under the carpeted areas and anywhere haevy objects are in contact. I would address those issues before they become even more of a costly project than what it's going to be now dependent on how much damage might already exist. Depending on your crawlspace construction and layout, you might be able to insulate the perimeter stemwalls and visquine over the soil area and up the stem walls without having to insulate the floor joisting. Less expense and a good way to keep any underfloor plumbing warm. Here's a link, there are many out there.

Here is a link that might be useful: crawl space insulating/moisture control

Thanks for the link. I read it and I may mention something to my landlord about it. I wonder if it is worth it to spend $ on putting insulation under the floor. If it would keep less cold out of the house, since the floor always feels like an ice rink-- and I will be living here a good few years.... I know it's a rental, but money to insulate vs. money on electric bill. Is it a DIY job? the video makes it look easy, but lots of "crawlspace" on the video -mine gets veryyyyy skinny towards the corners and the dirt is not even level. (house is a perfect square shape) oh, and my boards did finally dry. :)

Oh, yes. The wonderful experience of living in the bay area. Your problem is the bed frame has no ventilation. That is the problem with your mattress.

We sweat alot when we sleep. Even in the freezing cold uninsulated shacks around San Francisco, we sweat. All the sweat has to go somewhere. Most absorbs into the bedding and eventually down into the mattress. To dry properly, the mattress must have free airflow underneath. The memory foam REALLY needs ventilation, much more so than an ordinary spring mattress.

Do something to create an opening aroung the bottom of the bed. Have a carpenter cut the bottom, or build a simple pedistal.

Now that I think about it, I could help you. I live in Pacifica, where are you?

I could also give your landlord an estimate to insulate the attic and crawl space of that old house.

I got a nice tax credit last year for doing this to my house. I think I got about $1,000 tax CREDIT. All I had to show was my receipt from Home Depot for all the R-30 and R-19.

Send me and email: aidan@slac.stanford.edu

cheers!

I didn't think Thompson Water Seal would take so long to dry. It has been 24 hours, but it looks like it is going to take a lot longer. I just want to tell others not to do this the day before a party. It looks amazing. My deck is right out my front door. I noticed my dog's tracts across the deck; to go out side. I guess I'll be using my back door for a while. The temp. here has only been in the mid 60's. So I just have to be patient. I bought the clear with no color, but it made it plenty dark. But the wood grain does show thru. It's a good product. I just have to read the instructions. You know how us guys are. LOL

I first sprayed the deck with the Thompson's seal about 5 years ago. The first coat looked good and dried OK. I decided to put on a second coat, it was not dry for over a week. I read the directions and it said to put on only one coat! I called Thompson's for advice. They said a seal remover was the only way. They also advised that after the stripping that I not apply anything as the wood was sealed. I got on my knees and did as they said.

The second time about 2 years ago, I applied one coat and it worked fine.

I just did it a 3rd time this month, June 2018. The outer boards most exposed to sunlight did just fine but to make it uniform for the whole deck, I did the whole deck. The next morning, the deck was very oily, slick and sticky and I had some rain too. Rain did not affect the application or drying at all. I read the instructions again and it mentioned wiping any excess sealer off with a towel. I did that, even a couple times and it was still slightly sticky, but much better. Over the evening I was reading of some methods of removal; ammonia, simple green, mineral spirits, paint thinner, wax & paraffin removers, hair blower or iron (with towels) and wipe and even polar vs. non-polar (water base vs oil base). I don't know if any of them would have worked because the next morning despite rain and hail and 60 degrees, the deck was dry!

In short, I would recommend in the future, when applying it is a good idea to wipe the excess off right away with a towel (a rag). Kind of like applying Pledge to your wood furniture. If you are caught unawares the next morning as most of us are, I recommend getting on your knees and wiping it down 3-5 boards at a time trying not to leave any hand or knee prints, maybe 2 or 3 times. It will come off as a gummy or oily residue. This gets off the excess allowing for the deck to dry.

Been there,

Slow learner.