A4 On the polycrilic I use automotive Laquer primer and it works fine. I haven't found anything that will bother the polycrilic. I have been useing it for about 4 years. It is real userfrendly. Tha only problem is on thin balsa sheting 1/16 th. it will buckel the balsa beings it is water base.
On a sheated foam wing you don't have to worry about it.
If I am building a build up fus and wing and it gets thin, I will use Laquer sanding sealer to put the cloth down. Then put a coat of the polycoat on. It won't buckle the wood then. Rich