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Q.Recently I discovered that an inexperienced member of my crew mistakenly used finish paint instead of an actual primer to prime a fir exterior door. Now the door has two coats of a 100 percent acrylic house paint, but no primer underneath. Since it leads to a covered porch, the door is protected from the weather, but in the winter the south-facing doorway gets a lot of sun. I'm worried that paint adhesion may be a problem, but short of stripping the door I don't know that there's much I can do about it at this point. Should I worry?

A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., and co-author of Finishes for Exterior Wood, responds: Many light-colored woods like fir can be self-primed (using the top coat as the first coat) with no problem. I wouldn't anticipate any adhesion problems as long as the original wood surface was properly prepared, and sunlight should not be a problem, as the acrylics are the very best resins for protection against UV degradation.

But if the door were to be fully exposed outdoors, you would need the benefit of a proper primer first coat (one recommended by the paint manufacturer for exteriors); in that case, I'd recommend that you strip the door and start over.

In general, primers should be used whether the top coat is an oil-based or a latex-based paint, because they're formulated to have better penetration, adhesion, and water resistance than either type of top-coat paint.

A good exterior primer will seal in or tie up wood extractives so that they won't bleed through the top coat; since it's nonporous, it will also inhibit the penetration of rain or dew into the wood surfaces, helping to reduce the tendency of wood to shrink and swell.

When used on woods like redwood and western red cedar, oil-based, solvent-based exterior primers tend to have better tannin-stain-blocking properties than water-borne primers.

Water-based exterior primers are generally preferred for use on pine, Douglas fir, and most plywoods, because they are more flexible than solvent-based primers and are better able to withstand the dimensional changes of those wood substrates.

When primer is applied, be sure to follow the application rates recommended by the manufacturer. Use enough primer that the wood grain is obscured, but don't spread it too thick or too thin. A primer coat that is uniform, flexible, and of the proper thickness will distribute the swelling stresses that develop in wood and thus help prevent premature paint failure.