Using primer depends on the surface condition.
How to Build a Garage #8 Insulation, ceiling, wall finishes
If the drywall is new and unpainted, you should absolutely apply one coat of PVA (polyvinyl acetate) drywall primer first. PVA primer seals the surface of new drywall for durability and an even paint finish.
When paint is applied directly over new drywall, the paint soaks into the surface too fast, resulting in a poor finish.
When paint is applied over primer instead, the paint dries evenly and looks better.
If your garage walls are already painted with a latex paint, priming the whole interior is usually unnecessary, unless there are problematic stains bleeding through your paint.
A spray can of an oil-based primer sealer works great for getting rid of tricky stains. Products like Kilz, Preprite Problock, or Cover Stain, are all good choices for priming tough stains, but only the oil-based version, not latex.
If there are stains all over, rolling primer throughout is best.
Painting Over Oil-Based Paint
If the walls are currently painted with oil-based paint, the surface will need to be sanded and primed with a bonding primer, specifically a modified acrylic primer. Latex paint doesn't bond well to the hard, glossy finish of oil paint.
Painting latex directly over it, without prepping the surface first, can lead to paint failure. Sanding dulls the surface, so primer and paint stick well.
Another option is to simply paint the surface with oil-based paint to bypass the prep work.
Bonding primer to use over oil-based paint on garage walls:
- Sherwin Williams Extreme Bond Primer
- KILZ Adhesion Bonding Primer
- X-I-M UMA Bonder
Painting a Light Color Over Dark Colors
If the existing garage wall paint is dark, painting white over it could take several coats without primer.
Sherwin Williams Premium Wall and Wood Primer works well. The primer is thick, allowing the paint to cover well in one or two coats.
Sherwin Williams Preprite primer (latex) works well too.