Earl,
I am using some epoxy that I purchased from Fiberglass Coating Inc (
www.FGCI.com). It a 1 to 1 mix that is very easy to use. I am sure other epoxies can be thinned as well, I just happen to find a 1 to 1 ratio easy and I have used this product before with great results.
The epoxy is mixed per the instructions and then a thinner (I use acetone) is mixed in (about 2/1 - 3/1 epoxy to thinner) to make it more easily brushed and also to allow it to better penetrate the wood. When it hardens it is scuff-sanded a little just to roughen up the surface and then painted. This is referred to as encapsulation and is commonly used when building stitch-and-glue boats (although there the boat really is encapsulated, coated inside and out.)
You can also thin the epoxy by heating it but then it doesn't go as far when applying. You can, as Asianflava suggested, skip the thinning and use a less viscous brand of epoxy.
I don't claim to be an expert on this at all. I have used this method before and it worked well and will allow me to really seal the cheap plywood I used for my roof and strengthen it a little as well. Paint really seems to stick to it too.
Hope this helps a little. There seem to be a number of folks on the forum who have used epoxy for one thing or another that probably know more than I do. Also search the web for amateur boat building. This is what works for me and believe me I wouldn't spend the extra $$ if I didn't think it was worth it for longevity and waterproofing.
Ed