Hi!
Oracover or as it is known in the US Ultracote, is the best plastic film covering available, but it doesn't stick that good to sanding sealer (cellulose dope with filler material).
It's the same with all other plastic and tex films: they don't stick well to a cellulose doped surface.
A good filler material to use is polyester resin and brown microballons. Ultracote sticks to this as good as on balsa.

Regards!
Jan K
Sweden