Q: What is the basic difference between exterior and interior paint? The reason I am asking is that I have just painted some of my arbor in the backyard and, after I finished, I realized I had used an interior paint. This arbor is exposed to the weather. - Albert J. Sindlinger, Visalia, California
Michael's Advice: Paint consists of solvent, pigment and resin. It is the resin, which acts as a binderto bond the pigment to the wood, and also provides the protective film. The resins used in exterior coatings are more flexible than those used for interior ones. That's because outdoor wood moves more, thanks to wider swings in temperature and humidity. In addition, exterior paints employ resins and pigments that are UV stable. Many interior resins will yellow or deteriorate in sunlight, while some interior pigments can chalk or break down if used outdoors.
The same is true of clear finishes. Exterior polyurethane, whether oil-based or waterbased, is typically made from aliphatic urethanes which have a vastly higher resistance to UV degradation than the harder aromatic urethanes frequently used for interior polyurethane. Exterior finishes and paints may also contain mildewcide and fungicide to prevent the growth of mold and mildew. Clear coatings may contain ultraviolet blockers or absorbers as well, to lessen the impact of the sun on both the coating itself and the wood below.
You did not ask, but my guess is that you would also like some advice on whether you need to remove what is on there,or can simply cover it with something else to save it. If you used an exterior primer below the interior paint, you can go over the paint with a good exterior paint and should have no future problems. If, however, you did not use a primer (in which case, shame on you!) there is a good chance you will see peeling and delamination within a few years. Remove the finish either with a chemical stripper or by sanding, then use an exterior primer topped with exterior paint.
Q: What is the best formulation for chemically aging or pickling redwood? I know that fungus attacking redwood and turning the lignin gray is the process used by nature. I want to accelerate the process.
- Andy Anderson, Saint Helena, California
Michael's Advice: Whoa, let's back up a bit. Woods left unfinished turn gray, but it is not because of any fungus. The graying is caused by simple oxidation of the wood surface. Any finish will block this process, so to have it happen, you need only leave the wood unfinished. Secondary to the oxidation is the effect sun has on wood left outdoors. Sunlight will often make wood appear more translucent, changing dull gray to a silver-gray with a nice shimmer. Exposure to the elements will also make the surface rougher. Short of putting wood in a hyperbaric chamber, I'm afraid there is no practical way to speed up the oxygenation process so that it looks precisely like what nature does. Sad to say, the same is true of the translucent quality sun exposure imparts. That is not to say you cannot mimic the color or roughness with a variety of finishing tricks, but it is not so simple as flooding on a potion and walking away. Most chemical reactions that change the look of wood are slanted more toward adding color and making wood darker. One you mentioned, pickling, is a good example. It is a traditional chemical stain that uses nitric acid to turn softwoods darker. While that may look aged to some, it does not look much like oxidized gray.
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| Our author introduces instant (well, almost) aging using a wire brush combined with some loosely mixed white and black paint. |
| Click to Enlarge |
My favorite way to mimic grayed, weathered softwood is to first scrub with a wire brush along the grain to get a rough texture, then wash with driftwood color stain. I mix either oil-based or waterbased white and black pigments in their appropriate solvent, and stir lightly. Flood it on, wipe it off, and you will have a nice, gray driftwood color that lodges unevenly in the brushed recesses. The poorly mixed stain will color some areas more white and some more black, giving the wood a taste of the varying grays that nature imparts.