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Prep Steps for Outdoor Painting

Outdoor Living, Painting January 9, 2004 Matt Weber


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A fresh coat of paint is one of the most inexpensive ways to freshen-up your home’s exterior. But whether you plan to paint wood, masonry, metal or even plastic, preparing the surface before applying the topcoat is the first critical phase of the project. It may be a dreary procedure, but taking the time to correctly prep the job will pay off in smooth, durable coverage sure to last for years.

Paint is basically colored glue. Because it is designed to form a film, its inherent nature is to peel when it can’t adhere to a surface. Paint will, however, stick well to substrates that don’t shift beneath the paint. Metal of just about any type is an excellent substrate. Aluminum siding is a good example; paint can last for years on aluminum. Aluminum even expands and contracts, but the paint holds on. Wood and other “moving” surfaces that contract or expand due to moisture or temperature change may require some extra preparatory steps to minimize peeling.

             

Cleaning and Scraping

First, check the label on the can of paint you intend to use. It’s a safe bet that somewhere the label reads: “Apply to a clean, dry surface.” Before priming, make sure the exterior surface of your material is clean — free of dirt, grease, mold, mildew or any other debris.

For previously painted surfaces, start by removing any loose, peeling paint already present. Flat putty knives or “pull” scrapers work well for this. Pull scrapers feature shallow U-shaped cutting blades that help cut through old oil paints. Remove as much loose paint as possible, and wear goggles when scraping overhead to keep paint chips out of your eyes. Then wash and rinse the surface. Check the scraped areas again where the paint meets the bare surface. Likely, there will still be signs of paint film lifting slightly from the surface. If so, scrape these areas again.

Some substrates can be sanded smooth where the old paint meets the bare surface. For maximum paint adhesion on glossy surfaces, scuff-sand the surface before painting. But keep in mind, there are some situations where sanding isn’t a good idea. Some old, exterior house paints contain lead. Sanding this type of paint will generate toxic lead dust. If you don’t know the history of the paint you’re removing, you can contact local health officials for information on lead paint testing.

The next step is to wash the surface. Regular liquid dish soap may work fine, although it does produce a lot of suds. You may opt for a non-soapy detergent such as trisodium phosphate (TSP) or a TSP substitute to get rid of dirt and grease. Using the soap, rub the entire surface with a sponge or scrub brush to remove all dirt. If mold or mildew is present, use a mildew-removing solution, available at home stores like Lowe’s and Home Depot. Then quickly rinse with clear water from a standard garden hose to remove the dirty water, and let the surface dry thoroughly before priming. Allow porous surfaces such as wood or brick to dry for a minimum of two days before continuing.

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Remove any existing loose paint with a paint scraper and clean the subsurface throughly with water and detergent.

 

Wood Worries

Wood can be the toughest material to paint. Its tendency to expand and contract can take a toll on the strongest paint. And with paint as wood’s only line of defense, if exposed edges or seams aren’t painted, severe rot can develop within just a few years. All wood surfaces must be primed with an exterior wood primer.

Paint will adhere quite well to wood if the entire wood piece is primed before it’s installed. That’s why many wood sidings are available factory-primed, which is highly recommended for new homes or room additions.

To seal the wood completely requires painting and priming the cut edges, as well as the face of the boards, but few carpenters bother to do this. The cut ends are often where end grain is exposed, and this is where it is easiest for water to enter the wood. The absorbed water can migrate 5 to 6 inches away from the end of the board. This is often why you see the edges or bottom portions of trim boards flake. Do what you can to eliminate this kind of water intrusion.

Try using standard acrylic latex caulks to seal the end grain. Wipe the caulking with a moist sponge as soon as it is applied to produce a clean, smooth joint. If painting siding, caulk all joints between the siding and trim boards. However, do not caulk the bottom edge of clapboard siding. That small crack on clapboard allows the siding to move and allows vapor to escape.

For wood surfaces, Apply a rust-inhibitive primer to any exposed nails. Countersink the nails and fill the holes with exterior spackling compound.

Oil-based primers are excellent choices for wood projects because they penetrate deeply into wood fiber. This seals wood such as cedar and redwood that can produce bleed stains when coated with water-based products. Primers are a must on bare wood and any previously painted surfaces that exhibit blisters, flaking or any sign of paint erosion.

To accommodate for the movement of wood, it helps to use paint with built-in flexibility. Acrylic resins (paint glue) offer the greatest amount of flexibility. These paints resist getting brittle like older oil-based paints, and they can withstand years of substrate movement.

 

Surface Selections

Different substrate materials require different prep methods. Jobs that involve new concrete block, for instance, shouldn’t be painted until the mortar has cured for at least 30 days. For a smooth surface on concrete block, products such as Sherwin-Williams’ PrepRite Block Filler allow the topcoat to be more uniform and unbroken.

Other projects may involve poured concrete, stucco, masonry and cement-composition board. When newly installed, these surfaces must cure for at least 30 days and have no moisture problems before painting. Use an exterior acrylic masonry primer for these materials.

 

Concrete Floors

Before painting concrete floors, perform a moisture test. Tape a 2-foot square of plastic to the floor and wait about 48 hours. If the concrete discolors or there are any visible signs of moisture trapped in the plastic, the floor shouldn’t be painted. Otherwise, use a degreaser to thoroughly clean the surface. Then roughen the surface with a wire brush and acid-based solution to help ensure a strong bond with the paint. A mixture that works well is one part hydrochloric acid to one part water. After applying, rinse the floor with water and let it dry for two to three days. Allow new concrete to cure for at least 30 days prior to painting.

 

Metals

When it comes to prepping metals for paint, start by cleaning away any corrosion with a wire brush and sandpaper. Metal should be primed the same day it is cleaned. Oil- or alkyd-based metal primers that contain high quantities of rust-inhibitive pigments tend to get the best results when priming rusty steel.

Unfinished galvanized metal or aluminum should be wiped with a degreasing cleaner, rinsed and dried. There are both water-based and oil-based paints that work well for metals, but be sure to use paints that are compatible with your primer. Most manufacturers indicate what type of primer should be used for a particular finish coat, and vice-versa. 

There are also special paints available for painting the metal of central heating pipes and radiators. These special paints are designed to stand up to the high temperatures of these areas.

 

Aluminum and Vinyl Siding

For aluminum siding, first clean the siding with detergent, rinse thoroughly and let dry for several days. Remove any mildew, chalk or other debris; paint will not stick to a chalky surface. Replace any loose caulk. To prime aluminum siding, leading manufacturers recommend using a thinned metal priming paint. Try a mixture of one pint of paint thinner to one gallon of high-quality oil-based primer. And consider having the primer tinted to half the strength of the color of your finish paint.

For vinyl siding, follow the same cleaning procedure as aluminum siding. An oxygen bleach solution does a good job of removing mildew from vinyl. Oxygen bleach won’t harm the vinyl or the plants in your yard. Priming the vinyl siding is not necessary, but remove any rust from metal areas and coat those with a good rust-inhibitive primer.

Vinyl siding will warp from heat buildup. For this reason, very dark colors are not recommended. As a general rule, don’t paint vinyl siding any darker than the color of the original siding. And make sure your exterior paint is formulated to stick to vinyl siding despite the vinyl’s tendency to greatly expand and contract. Recent advances in paint composition offer new paint options that work great on vinyl.

 

Plastic

When the object at hand is plastic, to which paint doesn’t easily adhere, new products like Rust-Oleum’s Specialty Plastic Primer are formulated specifically for paint adhesion and durability on plastic. This gives home decorators the ability to personalize their plastic items with any of hundreds of paint colors, sheens or specialty finishes. Another company, Krylon, offers a new line of plastic finish paint called Fusion that is designed to bond to plastic without sanding or priming. Available in 16 color choices, Krylon’s Fusion plastic paint is designed to dry in 15 minutes. Both the Rust-Oleum and Krylon products are available in spray-nozzle cans for easy application to irregular surfaces such as patio furniture.

Clean the surface of any plastic item before spraying on the primer (or finish paint) to provide a tight, adhesive bond between the plastic and topcoat. These advances in paint technology make available a vast array of options to transform drab plastic planters, lawn chairs or other items into eye-catching accessories.

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Plastics? No problem. New products such as Krylon’s Fusion line of spray paints make it easy to customize lawn furniture, planetrs and other plastic items.

 

When to Paint

Weather conditions can have a great impact on how well paint holds up. The worst conditions are extreme heat or cold. Paint needs to chemically bond to the surface it is covering. This bonding can only happen if the solvent (water or mineral spirits) in the paint evaporates at a slow or medium pace. Water-based paints used on a bright, hot or windy day can result in the water evaporating too quickly from the paint. Plan your painting for cooler temperatures and try to paint areas after direct sunlight has left them for the day.

The trick to prepping a paint job is to plan ahead, to be patient and to complete these crucial procedures. Don’t jump ahead. Sacrifice instant gratification for the sake of long-term satisfaction, and your topcoat will stick around for many moons to come.

 

Sources

3M, www.3M.com, 800-494-3552

Carter, Tim, www.askthebuilder.com

Krylon, www.krylon.com, 800-4-KRYLON

Masterchem Industries, www.KILZ.com, 866-PRIMER-1

Rust-Oleum, www.rustoleum.com, 847-367-7700

Sherwin-Williams, www.sherwin-williams.com