Primers look a lot like paint, but have a higher concentration of solids plus an adhesive binder in them. It essentially seals the surface and provides a smooth and clean area for paint to adhere. In some ways, it’s more of a glue or sealant than a paint.
You'll want to Prime Your Interior Walls Before Painting IF:
Always prime your walls before painting if the surface is porous. The surface is porous when it absorbs water, moisture, oil, odors or stains. For example, brand new drywall is a very porous material. Both the paper that covers it and drywall mud are compromised by water or moisture when they are not sealed first with primer. This material will literally absorb your paint right into it if you don’t prime first.
Untreated or unstained wood is also very porous. If you’re thinking about painting over that old wood paneling in your study, or painting a set of shelves, you will want to prime first.
Glossy surfaces are hard for paint to adhere to. If the wall is covered in a high gloss paint, enamel or if the wood paneling has been shellacked, you could paint coat after coat and never have it stick.
If your walls have any stains on them (old water damage, smoke stains from cooking or candles, kid “art” projects), you’ll want to prime first. Since primer acts more like a sealant/blocker, it will seal these stains in so they won’t leak back or show through the paint.
If the previous homeowners or tenants were smokers, had pets, were victims of a fire or cooked strong smelling foods, the walls themselves have likely soaked up and retained those odors. A high-quality primer will seal in and eliminate odors and prevent them from returning.
You’ll mostly need to prime before painting when you’re going from a very dark color to a very light color.
