I didn't use a primer. I just wanted the enamel to make a harder surface and soak in as deep as possible. Sanding would change the shape of surface lines, if there are any of course. I used Durabond 90 and Sheetrock 30. Only because I had them around. I think the Durabond is a bit harder. Harder to sand on the walls at least. The back of a vibrating sander should shake out any bubbles. I just tapped it a bit with my fingers.