I'm pretty sure that at least for non-high voltage transformers, Tek used a black polyurethane varnish for impregnation. I recall long ago, I picked up a couple of expired gallon cans at the Country Store. I never used it for transformers, but it made a great restoring material for rusted steel. On my "rust-bucket" '68 MGB project, I stripped the interior, and washed it thoroughly, then coated the whole inside with this stuff. The metal was OK, except the floorboards were full of pinholes and some up to maybe 1/8 D, and spongy in some places. The goop was thin enough to soak into the spongy parts, yet thick enough to just slop on and cover everything nicely, even bridging the holes, and made it very strong.
Ed
