Martin, I'm thinking out loud here after reading your question.

I'm pretty new to this myself but isn't it true that unthickened epoxy is not just a coating, but it also saturates the wood to some degree? If that's true then isn't it likely there is still epoxy embedded in the fibers even though you've sanded through the coating? In that case it seems an additional single coat would be enough to encapsulate the wood in the sanded through areas, but maybe not enough to hold up to the wear and tear of use. If that's true what about putting a few coats just in those areas and feathering it in?

Again I'm not in any way an expert, thinking out loud, and hopefully not confusing the issue.

Hi Martin, 

i think its a reasonable approach.  just make sure your final coat is smooth (e.g., you are not creating drips that you then need to attack) and your final sand is not aggressive.

there are always places that are easy to sand through even with 220.

at some point, you just have to call it done....and it sounds like you are at that place.  and your strategy is a nice 'period' on the epoxy phase which will allow you to go to varnish work.

h

Unthickened epoxies we use for builds don't typically saturate wood (or glass!) fibers in plywood or solids but they do penetrate open grain in both when properly used, effectively binding fibers and epoxy into a matrix that makes S&G worthwhile.

There are other formulations that will penetrate deeper in solid wood but whether they can pass the glue layers in plywood? I don't believe so, at least not in the marine ply products we mostly use.

Adding a last coating of epoxy to areas sanded bare gives back some physical protection against water intrusion so it's a good thing to be doing before varnishing.

I'm a fan of the Scotchbrite family of abrasives myself over sandpaper for this kind of work. It seems less aggressive while also being easier to conform to shapes that otherwise might get sanded through when using paper. The key being when to know which agent will work best for the task at hand, knowing there's an alternative can be helpful.

   I really appreciate the thoughtful replies. I believe I am going to do two coats of epoxy but the first one will be only on spots I either KNOW to be bare or think COULD be. A light sand of those and then one thin coat on the complete interior. And I've heard about using scotchbrite before too and think i'll try that. 

Much obliged. 

   1. There are areas where sanding through the epoxy is easier some not so easy.  If you are getting a lot of sand throughs maybe you need to look at your sanding methods.  Go hand instead of power. Use less agression.  Check with the other hand for feel of sanding, smoothness bumps to changes in material, etc.  

2.  You know sanding the floor smooth and possibly the seats smooth in the JS II can be counter productive. You need a little anti slip texture.   

3. Through the years I've painted, varnished and oiled wood on boats that went in and out of the water.  Varnish can protect quite well  of those bare areas especially above the water line positions. 

4. Becareful with that coat of epoxy you intend to put on the boat not to drip ro run.  YOu'd need more sanding...........