Painting: Prepping a House for New Stain
In the fall of 1990, I applied two coats of a semi-transparent exterior stain to a house with new board-and-batten siding. The boards are Douglas fir plywood sheets and the batts are redwood strips. The stain contained mineral spirits, so it was oil-based.
On the shady side of the house, the stain is still sticky six years later. On the sunny sides, the stain has bleached out and taken on a chalk-like appearance. In "normal" moisture areas, large areas of the siding turned black with what looks like mildew.
How do I prepare this house for a cover-up of new stain or paint? -- R.G., California
: It would help to diagnose the problem first, before beginning the cover-up. You should contact the manufacturer of the stain you used. According to Lou Eggert, technical director of the Painting and Decorating Contractors Association, this problem requires an on-site visit. With so many variables involved, it is difficult to determine from your description whether this is a product failure or an application problem.
The stickiness could be caused by something oozing out of the wood or by a prior preservative preventing binding between the stain and the wood. And the dark spots may not be mildew. Eggert says that tannic staining from redwood and cedar looks exactly like mildew and even passes the bleach test.
Bleach will kill mildew and make it disappear, and it will do the same with tannic acid. The only way to confirm whether the spots are mildew or tannic staining is to look at a sample under a microscope.
Eggert also cautions that the stain may not be an oil-based coating. There are some latex coatings with mineral spirits in them. In California, thanks to stringent volatile organic compound standards, it is difficult to find oil-based paints.
There is one other thing going on here. Exterior semi-transparent stains are little more than thinned-down paint. They work about as well as you would expect thinned-down paint to work.
Now, how do you solve this problem? Begin with a good cleaning of the surface. Use either a power washer or scrub the whole house down.
Then let it dry. Eggert stresses this part especially. One of the possible causes for the initial problem is that the siding may not have been sufficiently dry when you first applied the stain.
The first coat should be an oil-based primer. "You've got to glue that stuff together," says Eggert. A latex primer just won't do the job. "Latex is better in many ways, but it has poor moisture tolerance," he says.
After coating with a primer, you can paint over with whatever you want, including latex. Make sure that conditions -- temperature and moisture level -- are appropriate for painting.
User Comments:
No comments added
Add your comment
Fill out the fields below:
|
