Took off old wallpaper and adhesive in the dining room, above wainscoting. The wall seems to be made of plaster, (possibly concrete behind it, built in 1929). There are imperfections on the wall, cracked spaces not too deep,... can I put Gardz sealer on it, then use a high-build primer, and then paint? Will that be enough to smooth out the wall imperfections to give it a smooth look? And if I use Gardz sealer, must I still use an oil based primer?