HI, not sure i would bother to be honest. Never really thought about it. Need to take care of heat, what ever you use might smell a bit when ever the transformer warms up. Just be very carefully you don't snag the windings or the connection to the taps!
I just used some thinned waterbased satin black. Some people do the paper coverings on the windings but I left mine. Try and mask off your serial sticker too if you can, although you can replace with new easily enough if it's too far gone.
Well done Carl.
On the amplfiiers restoration pages I go on it makes Pinside look like a pleasent chat over a pint of bitter.
As long as the lamination nut and bolts are tight then that is all I worry about in a transformer (we can not call them trannies anymore)