Hi Spidey:

Sanding between coats of varnish to a uniformly dull matte finsih breaks the molecular chains that bind the whole thing together, and does allow you some chemical bonding between coats. While it is true that there is some mechanical bonding between the next coat and the sanded surface, it is more important to sand to remove the glossy surface because there are additives in coatings that develop a really high gloss, and when that coat is cured,(or nearly so) then there is not only litle or no chance for molecular bonding, but a release agent on the surface. It is the gloss-developing additives. think Teflon or silicones...that's the sort of thing used to give high gloss. In the course of Doing Its Thing, adhesion is destroyed unless the surface is sanded to remove the stuff.

Varnish will not chemically bond to epoxy therefore it needs a stronger mechanical bond than is needed between the coats of varnish where a mechanical AND chemical bonds are present.