I think the 'COATING' you maybe seeing is:
Glyptal 'insulating Varnish (Paint)' the same stuff rodder's or eng builders coat the lifter valley on motors to aide oil return.
Where Eastwood or your local electrical materials Rewinding sho (i.e. EIS INC.)
Easwood link
http://www.eastwood.com/glyptal-red-enamel-set.html
Mfr. G.E. or "Spray-On" varnish.
The coating is used to preserve the alternator winding iron core laminations from corrosion. The product is avail in different classes (i.e. temeratures and colors ((Black,,,Red,,,Green). Avail in areosol or brush on.
Jim