Gypsum Primer / Sealer. A fast drying alkyd especially formulated for use on gypsum surfaces. Ideally suited as a primer / sealer prior to the application of a latex top coat. Available Sizes: 1-gallon: Zinc Chromate Primer. Based on a combination of anticorrosive pigments and alkyd medium. For painting all interior and exterior steel surfaces. Jun 02, 2009 · A primer serves several purposes, first, it’s a surface sealer for one, secondly, it gives the paint a bondable surface to chemically adhere to. Two separate processes. Sure, you can paint anything without primer, but it sure won’t look as good or lasts as long as a paint job that’s done right. Aug 25, 2018 · Summary of Paint vs. Primer: Use primer when you’re painting a porous surface or a surface that most paint won’t adhere to. Also use primer when you need to cover a dark color with a lighter one. Always cover primer with a top coat of sealer or paint. You don’t always have to use primer before paint.