Primers are formulated to penetrate, seal, and provide a good surface for the top coats to stick to. Use them over bare wood, Spackle, and epoxy, or over paint with a chalky, deteriorated surface. (If the paint surface is clean and sound, you can skip the priming step.)
Acrylic primers can be used on most surfaces, but on cedar or redwood, oil-based coatings are a must because they lock in these woods’ reddish-brown "extractives," which will leach out and leave behind rusty stains if the wood is primed with a water-based product.
Painters often tint primer close to the color of the top coat, but Wallis thinks that’s a recipe for "holidays," or missed spots. Instead, he tints his primer a contrasting color. "If I can see the color coming through, I know I need to apply more paint," he says. On the cottage shown in this story, he chose a gray-blue primer to go under a peach top coat.
If primer is sprayed on, "back-brushing" it immediately by hand will work the coating into every crack and crevice.