Nicholas,

I battled with the same problems of foam brush falling apart and too thick a varnish for most of the two boats I built last year. More thinning would only accelerate brush self-destruction.

The solution to my particular situation (danish-made Hempel varnish) was to get good brushes with dense bristles and no thinning whatsoever. I also warmed the varnish and brushed from dry to wet to avoid applying a double layer of varnish at the wet edge.

Not thinning the varnish made it behave much better (film formation) and allowed me more efficiency (pouring back the remaining varnish through a filter into the can).

I know that most people are very happy with foam brushes and thinning. But just get one good bristle brush and try it the easy way explained above, you might like it.

G'luck, Pascal