The best source for finishing infomation I have ever found is Bob Flexner's "Understanding Wood Finishing". Read it and you will learn more than you ever wanted to know about the subject. I can't over stress how usefull of a book it is. Most libraries should have it.

In his book as I remember he does recomend thining the varnish, and depending on the viscosity sometimes for all coats. This said I know some marine varnishes have special thinners that seem to work much better for them than mineral spirits.

Bart