When I worked for the Transformer factory we used polyurethane on copper wire windings. NEW transformers were very slowly lowered into a tank of polyurethane for about 12 hours so air had time to escape while polyurethane replaced the air. Then transformers were drained and baked in a oven until polyurethane was dry.
When I make transformers at home on my lathe I hand paint polyurethane onto each layer of wire with a brush. Big wire air dries easy in warm weather about 1 week with no oven. Smaller wire does not dry as quick.
If your trying to refurbish and old transformer you might be ok to hand paint it with polyurethane, let it soak in then paint it several more times & soak in several more times.