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Primers and sealers

This article presents general information about primer, primer/sealer, underbody primer and White-Pigmented Shellac.


Primer adheres durably to a variety of substrates. It bonds successfully where paint alone may fail. Primer also helps to fill, or “surface seal,” absorbent areas on surfaces, allowing paint to spread evenly and dry uniformly. Uniform drying is essential for a consistent finish when applying glossed paint.
Note: A substrate may be an unpainted surface or surface with existing layers of paint.
Note: Gloss paint finishes include: matte, low-luster, eggshell, semi-gloss and full-gloss.


Primer/sealers combine the qualities of durable adhesion, surface sealing and stain sealing. Most primer/sealers can also be used as a “barrier coat” between layers of latex and oil-based paints. A barrier coat is necessary to prevent oil-based and latex paint substrates from meeting, which could result in poor paint adhesion and areas of peeling paint. Before applying a latex paint over a surface previously painted with an oil-based paint (or vice-versa), first apply a primer/sealer that can act as a barrier between layers of oil-based and latex paints. (ask your paint dealer for their best primer/sealer that can be used as a barrier coat)

Underbody or Undercoater

Underbody primers, also called undercoaters, are used to prime new wood before the application of trim paint. Underbody primers are thicker than other primers and dry more slowly. The slower drying process of underbody primers allows for greater primer surface penetration. It also helps to reduce, or level, brush marks. When properly applied, underbody primer has a smooth, uniform appearance with a minimum of visible brush marks when dry. Wood trim has a richer look when paint is applied over an underbody primer. Some underbody and undercoater primers have stain sealing ability.

White-Pigmented Shellac

White-Pigmented Shellac is used to seal the toughest stains. Seal wood knots and any stains that bleed though primer/sealer with White-Pigmented Shellac. Several coats may be needed to seal knots and stains. Follow manufacturer’s directions for drying times and re-coating.

This information can be found on page 100 of the book:

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

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