How to Choose Primer for Exterior Wood

We all know that proper preparation of exterior surfaces is critical to a long-lasting paint job, especially for wood. Not only is choosing a high-quality paint important, but picking the right primer is just as, if not more important. Using the right primer can double the duration of your paint job. There are quite a few primer choices nowadays: All-purpose latex primer, acrylic exterior primer, oil-based primer, shellac primer and breathable, oil-based primer. Choose the right on.

Difficulty: Easy

Instructions

1. For most exterior wood in reasonably good shape, pick a high-quality acrylic primer. It remains elastic and permeable so moisture can "breathe" through the paint skin. This will decrease the chance of the paint bubbling and peeling due to moisture in the walls.

2. Consider oil- or shellac-based primers for cedar or redwood. With these, you have to factor in problems with tannin staining. Tannin is a natural substance in the wood which will bleed through and cause yellow or brown stains in your finished paint job. It is water soluble, so it will bleed through any water based product. Oil and shellac primers will stop tannin from bleeding through. Some water-based primers claim to be able to stop tannin staining, although sometimes two or more coats will be necessary. Use your own judgment.

3. Ask at your paint store about breathable oil primers, which are formulated to allow passage of moisture from the house through the paint. (All water-based products, unless they are sold specifically as moisture-barriers, are breathable.) Breathable oil primers would be your best choice to help combat tannin staining some plywood materials, in addition to cedar, redwood.

4. Use oil-based primers for very punky, soft older wood. Oil-based primers penetrate much deeper into the wood fibers and create a more solid substrate to paint over.

Tips & Warnings

• Do not wait longer than 30 days between priming and painting. The longer the primer sits uncovered, the more grime it can accumulate and the harder it gets, decreasing its ability to bond well with the topcoat.
• It's generally advisable to get paint from a real paint store rather than a big-box hardware store. The store personnel are better informed, and there is a wider range of products.
• Shellac-type primers don't tend to hold up well outside in any area where they are exposed to weather, like windowsills or south-facing siding and trim.
• Oil-based primers are messier to use, harder to clean up and most people dislike using them.