Varnish is rather poor moisture barrier when comparred to epoxy resin. Yes, you can get some protection from varnish alone, but the exposed fibers have a slight tendency to act like a wick, and pull moisture into the areas around them.

When you get some moisture like this and certain hot conditions the moisture can evaporate, and the resulting steam will still be trapped under the plastic layers of epoxy resin or varnish. In a mild case you get some bubbles. In a major case you see some local delamination.

These types of problems usually take a long while to develop, if they develop at all, and are easy to fix.

But, you can save future repair time by coating the exposed glass with another thin layer (or two) of epoxy resin before you do your varnishing.

PGJ