The Steps for Painting Exterior Concrete

Testing

• Concrete naturally absorbs fluids, a characteristic called porosity. However, clear surface coatings and other factors can decrease porosity. Insufficient porosity prevents paint from adhering to concrete. If a concrete surface quickly absorbs a small spray or sprinkle of water, the slab is sufficiently porous for paint application. Additionally, concrete that contains too much moisture prevents proper adhesion. You can test moisture content by tightly taping a square of foil to the concrete surface and allowing the square to remain in place for approximately one day. If the interior of the foil is relatively free of condensation, the concrete surface is usually suitable for painting. Excessive moisture indicates an unsuitable painting surface.

Stripping, Etching and Cleaning

• Concrete paint generally will not adhere to existing layers of paint. Therefore, you must completely remove old paint and other surface coatings with paint thinners or strippers and scraping tools before applying new concrete paint. Additionally, unfinished concrete surfaces benefit from etching. Etching compound, typically a strong acid, chemically scars a concrete surface to encourage paint adhesion and absorption. Homeowners must completely rinse paint stripping compounds and etching compounds from a concrete surface before painting. Note that paint stripping and etching compounds are hazardous and homeowners working with these compounds must follow manufacturers' recommended safety procedures.

Repair

• Painters must repair concrete surfaces before applying concrete paint. Concrete repair includes patching holes and filling cracks. While repair mortar fills large holes and cracks, painters sometimes use synthetic sealants, such as masonry caulk, to fill thin cracks and expansion joints. If you decide to use synthetic sealants, you should carefully read the product's label to determine if the sealant is paintable.

Surface Coating Application

• Painting techniques for concrete resemble painting techniques for drywall and wood. Some concrete paint manufacturers recommend applying a concrete primer. Concrete primers usually require longer drying times than interior paint primers -- often eight hours or longer. After applying primer, painters apply concrete paint with standard painting tools, such as rollers, brushes or power sprayers. Note that manufacturers usually specify between concrete paint and sealer for walls and paint for walking surfaces; the latter provides more slip-resistance. Additionally, painters sometimes texture a concrete seal coat with brooms or stiff-bristled brushes to increase slip-resistance.