They are commonly vacuum-dipped, to get varnish everywhere. We specify that.
The heat-cured is the best type, although others do work. The varnishes used are intended for the application, and differ by heat rating, and heat class of transformer.
Glyptal is best used for exposed areas only, it doesn't soak-in well, although its OK as a varnish. You CAN pull it in with a vacuum, but I don't actually know about its heat resistance.....
Get it done by a motor shop if you can, that will be best.
The correct type varnish will not affect the winding insulation at all. But you are right to be concerned, insulating systems vary, and not everything is compatible.
Any transformer will have used a UL insulation system, and everything will carry UL recognitions and have a file number.
No UL file number? I wouldn't advise using it in a power transformer.
[This message has been edited by J Tiers (edited 03-12-2006).]
Keep eye on ball.