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Prepping epoxy
I built a new deck last spring with three coats of epoxy but didn’t get to the varnish. I’m going to varnish this spring and prefer to wet sand the epoxy. What wet grit and should I use a RO sander? Will it differ if I decide to use Epifanes PP or gloss?
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That looks lovely, nice work!
My advice here is to sand coarsely but to not sand it back too far. You want to leave as much of that epoxy in place as possible to maintain a vapour barrier over the woods surface. If you break through to the wood in certain areas and then press on with varnish it begins to defeat the object of epoxy coating.
I would use 80-100g paper and sand entirely by hand, wet is a good idea. Don't aim to get your surface flat at this early stage. You are simply looking to scratch up the epoxies surface, de nib any major lumps but ultimately provide a secure key for the varnish that will follow.
Build around 6 coats of Epifanes PP and after that use this product to begin getting the surface flat and fair. This is when you can break out the RO sander.
Once you have got the surface nice and flat go over to your Polyurethane clear, depending on the destined environment for the boat build 6+ coats of that and you'll have a killer finish there!
When applying PP over the epoxy do the first coats need to be thinned as you do with traditional single part varnish over wood? My thinking is that the thinning is to get the varnish to soak deep into the wood grain, but if applying over epoxy this isn't the case and so I can skip the 50% 25%, 15% steps.
Not as much no, you don't need that deep penetration of the wood as the epoxy has already formed that bond. If you are using PP I would thin the first coat on to epoxy 25% just to help it get right into those surface scratches and then go on with no thinner from there onwards.