Thread: Plywood Floor - to Seal or not

Hi Karsten

I don't think paint or varnish will cut it, especially as its going to be perpetually wet. I'd be coating the ply with fibreglass resin (both sides and all edges). And I wouldn't be worrying about marine ply either, just good quality 15mm CD or flooring grade. Eventually it will get water ingress no matter what you do, however the fibreglass resin will give you more years of use than paint or varnish. I also recommend sprinkling anti skid material on the resin of the upper face before it goes off. A resin surface can be slippery when it gets wet. I just used fine sand.

You may have by now gathered I have faced the same problem.

If looks of the floor are an issue you can cover the resin on the upper face with flowcoat tinted to whatever colour you like.

Hope this helps

Regards
Twosheds

P.S. BOAT = Bring Out Another Thousand

Two sheds is right on the money here in terms of best treatment of your ply decking.

Further to that the 'Big Green Shed' had recently AA Marine ply in 12mm for about $80 per sheet. I bought a few and it is of good quality and from a proper source. Other outlets generally ask double that price!. With the non-slip you I use an old salt shaker which gives a more even spread of sand/grit rather than trying to sprinkle by hand. Another economical method of non-slip is a product called kiwi grip which can be obtained from Whitworths. Easy to use and actually works.

C.R.

Thanks for the input, TwoSheds and CoolRunnings. Yes, just to confirm, appearance is of zero concern, and the non-slip makes good sense (I've done the sand/ sugar/ proprietary etc. in the past).
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With using the fibreglass resin (and/or flowcoat), since it sets as a pretty hard layer, I have to ask whether it will be liable to separate from the ply once it's wet and the boards flex a bit?
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The support beam span widths are such that the 5/8 ply flexes a bit underfoot, but I don't want to go thicker since I really wanted to keep the total weight down (boat needs to plane easily). For that reason it had crossed my mind to use a soft water-based acrylic coating that "might" stay better bonded to the wood, but I haven't seen others use it in this way.
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In the distant past, whenever I've covered plywood with fibreglass cloth and resin, once the ply gets wet and flexes, or gets cycled by daily sunshine, the bond between the grp layer and the ply had broken down. Won't it happen quite quickly here with the wet, the sun and flexing? Or should the aim be to make the resin coating quite thin, like a single coat of thin resin only?

I used BoteCote epoxy resin to seal both faces and all edges of my marine ply flooring. I used two coats and as I applied the first coat I warmed the plywood with a heat gun to get better penetration of the resin. This worked well because as I applied the resin I could see thousand of tiny air bubbles gassing off out of the plywood. Once I installed the floor in the boat I sealed all screw holes with Sikaflex 291 and used it also to seal all around the edges where the floor met the inside walls of the boat. When all the sealant was dry I gave the entire floor two coats of a non-slip paving paint. I did all this about four years ago now and the boat is always moored at a jetty in salt water. The floor is pretty much as good as new in that the paving paint hasn't worn or peeled, and no water has gotten to the plywood.

Your "fellow boater" is absolutely correct, in that an un-encapsulated hunk of plywood will eventually absorb moisture and cause issues. This would be particularly true of a mechanically fastened piece, like that in an aluminum hull.

There's a couple of approaches you can take. The first is to assume this is going to be an issue, so allowing some movement (wood swelling, etc.) and fasten it down with this in mind. This would likely include blind nut and rivet-nut attachments from the plywood to the aluminum, so it can move around a bit, but can be tightened down or removed occasionally as required. The other approuch is to encapsulate the plywood with epoxy (minimum of three coats) so moisture just can't get in any more. This would include "bonded" fastener holes and possibly more rivet-nuts, etc.

Conversely, you could use (I'd recommend it) an inert material, such as Cossa board, instead of plywood. It's generally a little lighter than plywood and its not affected by moisture, so you cut it up and just bolt it down. This is a more costly approuch, but can be bullet proof if desired. HDPE and other materials can do this job as well, though there's much to be said about Cossa for this particular application.

As a rule nonslip paints don't last long and wear out pretty quickly. It doesn't matter what the application technique is or the particulates employed within the coating. The best textured surfaces are applied with resin (epoxy) and this texture remains with the encapsulation, not the coating stages of the project. You can paint it, repaint it, run a chemical stripper over it at a latter point if necessary and the texture remains. It requires more planing and prep, but looks and performs much better.

The image attached is a texture job I'm in the middle of doing. You can see the "waterways", the textured areas, etc., but it's not finished. It is yet to be prepped for paint, which required another coat of resin and some additional sanding, etc., for paint. Because (again) the texture is part of the encapsulation process, not a coating process, it'll always be there. All I did to get this was tape off the waterways and apply a slightly thickened resin, to the exposed areas with a roller, which creates a textured pattern. When dry, the tape was pulled and the raised textured areas were coated again with straight resin to soften the texture a bit, then prepped for paint.