By the Paint Quality Institute
In order to get the best exterior painting results, it's often necessary to use a primer or sealer before applying the paint. But deciding when to use a primer or sealer can be confusing.
All top quality exterior primers help paint adhere better to the surface that's being painted. They do this by making the surface more uniform for the top coat, so that when the paint is applied, it can get a better "grip" than it would on the bare surface.
Primers also help give the finished paint job a more uniform appearance in terms of color, hiding and sheen. This is especially true when the surface being painted is porous or is uneven in porosity.
If there are stains on the surface being painted, some primers can help prevent them from "bleeding" through the paint and ruining its appearance. Not all primers have stain-blocking capability. If you are interested in this feature, check the label to make sure the primer you purchase is the stain-blocking type.
Experts say that almost any exterior painting project will benefit from use of a top quality primer, but there are certain applications where a primer is essential: namely, when painting new wood, bare stucco, or any surface that has not been previously painted. You should also use a primer when repainting an uneven or deteriorated surface or a surface that has been stripped or is worn down to the original material.
Certain applications call for a specific type of primer. The Paint Quality Institute offers these guidelines:
- Wood: On new, unpainted wood, use a quality acrylic latex or oil-based exterior primer. While quality latex primers will block stains from wood such as cedar or redwood, use an oil-based stain-blocking primer if the boards are particularly dark or highly prone to staining. Always prime and paint bare wood within two weeks of its first outdoor exposure to keep fibers from deteriorating and reducing adhesion.
- When painting weathered wood, first scrape and sand the wood thoroughly, then apply a quality exterior latex or oil-based primer. When working with previously painted wood, scrape off all loose paint and feather-sand rough edges and bare spots before applying the primer.
- Stucco and Masonry: On new masonry, or older surfaces that are very porous, use a latex masonry sealer or primer. If you are repainting masonry, at a minimum, spot prime where the old paint has been removed.
- Aluminum or Galvanized Iron: First remove any white powdery oxide, using a non-metallic scouring pad or steel wool. Rinse thoroughly. If steel wool is used, every residual particle of it must be removed. Then apply a corrosion-inhibitive metal primer.
- Ferrous Metals: If there is any rust, remove it with a wire brush, then rinse and allow the surface to dry before applying a latex or oil-based rust-inhibitive primer. Two coats of primer will provide added protection against future rusting.
The Paint Quality Institute, sponsored by Rohm and Haas Company, is an educational resource for consumers, professional painters and sales people on paint and paint-related coatings.
