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Spot priming and full coat priming


Spot priming is the application of primer or primer/sealer over a portion of a surface. A surface is often spot primed where it has been stained or repaired. Wood surfaces with knots are often spot primed with White-Pigmented Shellac to cover just the areas with knots.

Full coat priming is the application of primer or primer/sealer over an entire surface, such as a ceiling, wall or door. Surfaces where a decorative painted finish, faux finish or painted mural will be applied should always be full coat primed.

Argument for full coat priming

Slight variations in the color and sheen of applied paint can occur overed spot primed areas. These variations are possible when paint is applied over differing substrates (one being the existing paint and the other being the spot primed area, or areas). The existing paint and the primer or primer/sealer used to spot prime may accept the color and the sheen of the finish paint in differing ways. For best results, the entire ceiling, wall or trim surface should receive a full coat of primer or primer/sealer when individual spots have been filled or repaired.

Possible exceptions

As there are often exceptions to every rule, the full coat application of primer, primer/sealer or White-Pigmented Shellac may not be necessary, or practical in every situation. One possible exception to this rule is the application of White-Pigmented Shellac to seal wood knots before the application of trim paint. I have not seen a variation in the finish of trim surfaces when two coats of top-quality latex or oil-based trim paint are applied over a wood surface that has been spot primed with White-Pigmented Shellac. Your experience may be different. Another possible exception may occur on a surface that has had one or only a few small repairs. It may not be worth priming an entire wall or ceiling for the sake of a filled nail-sized hole or small indentation. One last example concerns the application of White-Pigmented Shellac spot primed over a ceiling or wall surface where a stain has bled through an oil-based or latex primer/sealer. In this rare occurrence, it may be advisable to prime and seal the entire ceiling or wall surface with an oil-based or latex primer/sealer after spot priming is complete. Primer/sealer is used in this situation so that the affected surface and the surrounding room surfaces have the same substrate for paint application.
Note: Surfaces with multiple repairs of indentations, nail pops, holes or cracks should always be full coat primed.

This information can be found on page 104 of the book:

The Homeowner's Guide to Surface Preparation for Interior House Painting.

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