ASK THE INSPECTOR: Sandblasting discoloured cedar a no-no

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This article was published 01/05/2010 (5635 days ago), so information in it may no longer be current.

QUESTION: We have exterior wavy-edge cedar on our home and garage. Over the years it has started to darken or in some spots turn black. We are wondering what is the best way to clean it as we would like to re-stain with a clear color this year. I was thinking of hiring someone to sandblast the buildings. Is this a good idea?

Jennifer Stockmann

ANSWER: Most wavy-edge cedar siding produced in the past was high-quality lap siding, often made from clear or almost knot-free cedar with a rough surface. It is indeed worth trying to save this material and renew the finish, but you must take care not to go overboard or you may damage the soft wood. My answer to your questions will prevent this from happening, if you follow the advice provided.

Sandblasting is a method of cleaning or renewing the surface of older material by bombarding it with fine sand under high pressure. This is often used on older brick, concrete, or masonry surfaces that can withstand the highly corrosive nature of this process. It can also be used on older wood, but is usually reserved for hardwoods or old fir beams and columns that have substantial thickness and strength. To my knowledge, it is not used on soft wood siding, like cedar or pine, because that material will not be able to withstand the force of sandblasting without damage. Fortunately, there is another similar process that may be ideal for your purpose.

Instead of blasting the deteriorated surface of your old siding with sand, you should be able to use a pressure washer that uses water. These devices are readily available for either purchase or rent at a moderate price. Since they only use water to remove surface finishes and dirt from the siding, other than covering bushes and vegetation to prevent damage, there are no negative effects on lawns or landscaping. Pressure washers are now in wide use by painters and others for preparation of a variety of surfaces prior to painting or refinishing, due to their ease of use. They are normally available in either electric or gas-powered models and require a lawn faucet or other water source, which is usually present on the outside of most homes.

While the water will do little to damage anything outside the home, care must be taken not to spray too directly to the siding, to avoid damage and moisture intrusion though the walls of the home. Windows, doors, and seams in the siding may be areas where water can easily penetrate the building envelope if applied directly under high pressure. Using the pressure washer at an angle, at a reasonable distance from the walls, should prevent these issues from happening. Recently, I used a pressure washer on my cedar deck at the cottage to remove old finish and dirt. I was surprised at the relative ease in removing actual fibres from the boards, if care was not taken to proceed slowly and at a safe distance from the surface. The benefit of pressure-washing your older rough siding is that the surface should be almost perfectly prepared to receive the new finish, once it has dried. You may have to do some minor sanding of rough areas or ones with excessively raised grains, but otherwise minimal preparations should be required.

While cleaning the surface in the manner suggested may be the most cost-effective and benign method, it is not the only one available. Use of chemical paint strippers may also be an effective way of removing the old finish. There are exterior strippers that are safe for plants and grass, but the cost may be prohibitive. Also, extensive scraping and sanding may still be required after applying the paint stripper to remove heavy finish material. Because this method uses corrosive chemicals, there can be areas of the siding that may require multiple coatings and still may appear unevenly cleaned when complete.

Once the siding has the remainder of the old finish and stains removed, there are several options for refinishing. The first thing to know is that the grey and then black discolouration of the wood is not normally due to rot or deterioration, but from the effects of the sun. Ultraviolet (UV) light will turn the wood a nice light grey, initially, but this can progress to dark grey or black if left to progress. If the discolouration is so advanced that the wood does not look relatively uniform in colour after pressure-washing, you may have to pretreat the siding prior to finishing. There are paintable materials available that replace the natural chemicals in the cedar, eliminating the grey and bringing back the natural tones. These are only effective on untreated wood or once all the old finish is removed, which should be possible after pressure-washing. These coatings are often costly, so they should be only used if the siding is quite badly discoloured.

The final thing to address is the material selected for refinishing. While you may desire a natural or "clear" colour, as you suggest, that may not be possible on older wood. Because further discolouration will be caused from UV exposure, selecting a finish with high UV resistance is critical. Most of the products with this built in have a substantial tint, which helps in keeping out the harmful rays of the sun. There are many products designed specifically for cedar that will give you the most natural look, but don’t cheap out. This is one area where you normally get what you pay for and the pricier stains or finishes will last longer and give you a more natural-looking result.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.mb.ca). Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.

trainedeye@iname.com