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Jan Rowland

Now, you are ready to impregnate the finished transformer in Formvar varnish. You might buy a gallon or two from that local transformer shop, or, they might tell you who their supplier is, and you can get fresh stuff from them. Formvar is special in that it takes forever to harden "in air", but when heated, it cures in half an hour. You might want to get the supplier to tell you how to handle the stuff you get.

You have wound the coil "dry" with only copper, insulation, and air "in there".  You will want to insulate the windings, and fill in all the air holes, and basically stick it all together, not only for insulative purposes, but to reduce audible humming when the transformer is in use, if not wound well, nor the laminations stacked well.  Some shops will just hang their completed transformers by the leads, or by wires temporarily through unused holes in the laminations, in a vat of Formvar, at room temperature, for 24 hours, or even less.  I was once told a 24 hour soak is just as good as a vacuum soaking, but I sawed-up a recently made transformer I had soaked for 48 hours before cooking, and it was DRY, deep inside!