When Switching From Oil- to Water-Based Paint, the Key is Prep Work

QI've always used oil paint on the exterior of my house, but now, because of environmental concerns, stores are beginning to carry only water-based formulas. I'm worried about the complication and expense of painting water-based over oil.

ADon't worry. It's usually possible to recoat oil paint with water-based paint successfully and wind up with a better-performing topcoat as a result. But you can't skimp on preparation.

The main complication stems from the fact that oil paint continues to harden over time, while water-based paint, which incorporates a plastic resin such as acrylic or vinyl, remains flexible. If you have an old house with many paint layers, the deepest paint is now quite hard and brittle. If you add a flexible layer that bonds well, as paint is supposed to do, the brittle layer can't keep up as the wood below and the new paint above shrink and swell with changes in temperature and humidity. In the worst cases, the old paint loses its grip and everything comes off in big strips.

This rarely happens, however, unless the old paint is quite thick. And when it does occur, the old layers were about to fail anyway. Adding new paint may accelerate the peeling or make it more noticeable. Instead of flaking off in bits, it peels in sheets attached to the new paint.

So how to proceed?

First, test how well your existing paint is sticking. Clean a patch of wall in an inconspicuous spot where the paint looks worst. With a utility knife, carefully cut a large X through all the paint layers but not into the wood. Thoroughly press on a strip of duct tape, then yank it off. If the tape is clean or shows paint flecks just along the scratch marks, you can safely switch to water-based paint. If all the paint comes off, the underlying layers are almost ready to peel. If just some layers come off, a previous painter didn't prepare the surface properly and the top coats are barely hanging on.

With either bad scenario, you will waste your money if you repaint without first removing all the old paint -- a costly process, especially if the old paint contains lead. You might want to coax a few more years out of the finish by just washing the walls and touching up the paint. Expect an inexact color match, even if you use leftover paint from the same batch now on the walls, because paint fades in the sun. If you need to buy touchup paint, stick with oil, which you can still legally buy in quarts. Also attend to any underlying cause of peeling paint, usually excess interior moisture or a leaking gutter.

Assuming the X test shows your existing paint is in good shape, you can confidently switch to water-based paint. If your house was built before 1978, begin by testing the paint for lead. If the test comes back positive, hire a professional trained to deal with lead hazards, at least for the prep steps, or make sure you know what to do. Some municipalities regulate this work through special permits.

If there is no lead, clean the paint with a power-washer, which you can rent, and a cleaner, such as a TSP substitute. (Avoid TSP itself. Besides releasing phosphate, which can run off and promote over-abundant growth of algae in streams and ponds, TSP can leave a crystalline residue if it isn't rinsed thoroughly, says Tim O'Reilly, who oversees primer paints for the Zinsser Co. When paint doesn't bond properly to old paint, these crystals are often the culprit, he says.)

The next step is to scrape off all peeling paint. Catch the pieces on tarps, and lightly sand edges of peeled sections to eliminate sharp ridges. Painting manuals used to recommend scuffing up all of the old paint to give the surface "tooth" so the new layer would stick better. But today's top-quality water-based primers stick fine without sanding as long as the surface is completely clean, O'Reilly says.

For the past several decades, most paint experts advised coating the entire house with an oil-based primer before switching from oil- to water-based paint. But water-based primers have improved so much that today you can use them except for a few special circumstances. The two most common situations where you still need oil primer: rusted nail heads, which need to be spot-primed with oil before the entire wall is coated with a water-based primer (after the oil primer has dried), and especially dark new cedar or redwood. Water-based primers labeled as effective against tannin bleed are fine for stopping moderate color transfer from these woods, as long as you allow extra drying time, usually 24 hours. To determine whether you're dealing with moderate or severe staining, coat a test section with water-based primer, wait 24 hours, then brush on the finish paint. Tannin stains typically move into the water-based primer, but if they transfer into the final paint, you need to use an oil primer.

Once the primer is on, treat your house as if it were painted with water-based paint from the beginning. Because preparation is usually the most costly part of a paint job, consider applying two topcoats. With only one preparation job, you'll double the life of the paint.

Find detailed advice about how to proceed with lead paint in "Lead Paint Safety: A field guide for painting, home maintenance, and renovation work," available at http://www.hud.gov/offices/lead/training/LBPguide.pdf . Or type the title into a Web search tool, such as Google.)