I've used all kinds of varnish and love Epifanes and it's the only kind I use.
Are you using the Wood Finish build up formula or the Clear Gloss finish coat. I never thin the Wood Finish by more than 20% on the first coat and not at all after that, putting on about 4 or 5 coats, all within 24 hours of each other, then sand. I use Epifanes own thinner. Next I put on 2 or 3 coats of the Clear Gloss sanding between each coat. I never thin the final coats.
I also have found that using a Black China bristle brush works best for me(and don't make it to thick a brush). They hold just the right amount of vanish and will not let you put too much on. Problems always come from too much varnish, not too little.
On this project I am using the Gloss Clear over an existing varnish. The old finish is really good so I sanded for adhesion and to remove blemishes etc. I only intend to add two coats on the inside of the boat.
I have Epiphane thinner that I will use to thin the first coat inside the boat. I plan to varnish the decks, rails, transom etc. (it's a Car Topper) unthinned, two plus coats.