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Chalking causes paint to peel off

QuestionI had my garage painted by a painting contractor about a year ago who used a medium grade acrylic latex paint. The siding appears to be T1-11. Can you tell me why the paint comes off the siding in big sheets? I'm sending photos so you can see that I'm not making this up. It really is coming off in big sheets. Thank you for any advice you might have.

AnswerThanks for the photos—they really helped me to understand the problem, and I can see you're not exaggerating! The garage siding does appear to be some form of T1-11, which is usually rough to the touch and is composed of distinct plywood layers with a cedar outer skin. However, it's easy to mistake similar products made of either wood fibers or wood chips for T1-11. To determine if it's T1-11, use a mirror to check the bottom edge of the siding for the plywood layers. Also, if the garage is unfinished, you should see a stamp on the backside of each sheet. The type of siding will determine the type of pre-paint preparation that is required; contact the siding manufacturer for their specific guidelines.

It's clear from the photos that the paint is not adhering to the siding's surface. This normally occurs when the old paint film is covered with a layer of "chalk." Chalking will prevent the new paint from adhering properly.

Chalking is a property of the paint and not of the wood surface and is confirmed by an obvious white deposit on your hand or a dark cloth after wiping the paint film. As the paint surface deteriorates from weather, the paint film breaks down, releasing pigment particles which act like a fine powder on the paint's surface. Most paints chalk to some extent, and that's OK because chalking cleans the painting surface and is a normal consequence of weathering. However, when the surface starts to discolor or the paint film prematurely fades through excessive erosion (some erosion is normal with colored or tinted paint), it's an indication that the surface needs careful preparation before painting over it.

Some causes of chalking include paint spread too thin or thinned too much; use of low-quality, highly pigmented paint; an indoor paint used on an exterior substrate; or the use of epoxy paints prone to chalk in direct sunlight.

I'm not pointing any fingers, but it appears that the garage wall's surface was not properly prepped to receive the finish coat. If you have T1-11 siding with a cedar skin (the skin could be of southern pine, Douglas fir, hemlock, or western red cedar), I recommend a top-quality oil-based primer both to provide a good bonding substrate and to prevent the natural coloring agents in the T1-11's cedar skin from migrating through the coating and staining the surface. If your siding is not T1-11, then a good stain-blocking acrylic latex primer will work, although I personally and professionally prefer the oil-based primer. Either way, proper preparation and a good primer are essential for the life of the finished coat.

If, in your case, the chalking was obvious before the surface was painted, it would have been a good indication that thorough preparation was required to ensure a good bonding surface for the new paint. Basically, preparation involves scrubbing with a detergent solution to remove all chalk deposit and dirt followed by a thorough rinsing with clean water. Use a stiff nonmetallic brush—not a wire brush as metal fragments can break off and rust, leading to stains on the siding's surface. The surface then needs to dry completely, which can take some time depending on whether the siding was cleaned by hand or a power washer, the time of year, and/or if there are areas where water can easily penetrate. It can literally take a week or longer for the moisture to work its way out. Finally, the siding is ready for primer application followed by the finish paint.

A good detergent to clean the siding would be TSP (trisodium phosphate). Follow the manufacturer's recommendations on the package. Best results are achieved by scrubbing with the grain of the wood using a medium brush. When the surface is clean, rinse with fresh water from the garden hose. If TSP is not available in your area or its use is prohibited because of ground water regulations, your paint dealer can suggest an environmentally safe non-phosphate product.

The APA-The Engineered Wood Association recommends testing the completely dry surface after cleaning and before primer application. Use a tape, such as duct tape, which has a great adhesive. Attach a small piece to the surface by applying pressure with your fingers. Then remove the tape with a quick pull. If the tape does not remove the finish or does not come off easily, then the surface is satisfactory for repainting. If it comes off immediately, then the surface needs to be cleaned, dried, and tested again.

To learn more about the types of finishes to use with siding, go to www.apawood.org and click on "Publication Store." Then type M335 into the search box. You'll access a (free) downloadable .pdf file that gives more details on finishing plywood siding for protection and appearance.

The real bottom line here is that prep work is the most important part of the project, and in the long run, it costs less—in both time and dollars—to do it right. With apologies to Thomas Edison, I believe that a good paint job is 99 percent preparation and 1 percent application.

Copyright © 2004, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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C.R.S., Inc. · Spokane, Washington · USA

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