A properly painted concrete block wall will be virtually maintenance free for many years, but the same wall not properly painted can be a maintenance nightmare. There are just a few critical differences between a good and a bad paint job.

The main considerations you need to think about are new and unpainted, condition of existing finish and type of concrete block. All of these will require similar approaches when preparing the block for the protective finish coat.

You will encounter two types of block, smooth and split-face. The difference of texture will drastically alter the amount of primer and finish paint needed. Count on split-face block needing twice as muck primer and paint when compared to smooth block. Exterior block surfaces can be either, but interior block surfaces are always smooth.

Preparing the Concrete Block For Painting

Painting concrete block walls always starts with the removal of dust and dirt, efflorescence, and all loose or peeling paint. Both interior and exterior block surfaces require the same attention to details.

Efflorescence Deposits

Plain water will remove loose paint and dirt but efflorescence will need chemicals to be neutralized. Efflorescence is a white powder that can form on the concrete block and is caused by water infiltration inside the block.

Can be caused by failing exterior paint, a leaking roof or metal cap flashing. Use acidic masonry cleaner to remove this unsightly mess. On interior surfaces use a scrub brush to fully remove stubborn deposits. While pressure washing the exterior inject the masonry cleaner into the washer and use a scrub broom on persistent areas.

After removing the efflorescence deposits find out were the water leak(s) came from and make any repairs necessary.

Interior Preparation

New unpainted concrete block walls need to have all dirt and loose mortar removed prior to priming. Painted walls will need all loose and peeling paint removed. Lift off the peeling paint with a stiff putty knife or paint scraper. Then scrub the area with a wire brush.

Also, any greasy or oily deposits will need to be cleaned with TSP, trisodium phosphate, and primed with a stain blocking primer.

Exterior Preparation

The first step when preparing concrete block walls is pressure washing. Use a commercial style pressure washer and at least 2500 psi. A suitable spray pattern is the 15° degree tip held 12 inches from the surface. Any remaining loose paint will need to be removed manually.

During the preparation pay attention to any loose mortar and large cracks. All repairs need to be done after the pressure washing.

Pressure Washing

A good washing will save a lot of time later. Don’t rush, it’s easier to pressure wash than manually scrape the loose paint.

• New and Unpainted – All your after is removing any dust, dirt and loose chunks of mortar. Mild pressure, 1500—2000 psi, is all that is needed. Work in a horizontal, back and forth, pattern. Start washing at the top and work your way down the concrete block wall.
• Previously Painted – Most of the prep-work involves removing all loose or peeling paint and fully evaluating the condition of the block. The pressure washer can remove most, if not all, of the loose paint. To remove the majority of peeling paint with a pressure washer use 2500-psi and 15° degree spray tip, typically white. Hold the tip 12 inches from the surface and work in a horizontal, back and forth, pattern.If any paint peels during the washing continue following the edge of the loose paint. Try to inject the water underneath to lift as much as possible. The spray tip can be within 6—8 inches of the surface to concentrate the water pressure. Follow this procedure with a vertical washing pattern.

In order to provide the most adhesion and best performance, all caulking needs to take place after the application of primer. You have many choices, as which to use.

The best types of caulk are elastomeric and polyurethane. Use non-sagging polyurethane for expansion joints and elastomeric around windows and doors.

Materials needed for caulking expansion joints are foam backer-rod and non-sagging polyurethane caulk. The backer-rod needs to be slightly thicker than the expansion joint, example- a 3/4 inch joint needs 1 inch foam backer-rod.

Insert the backer-rod 1/2—3/4 of an inch below the surface. Polyurethane sealants are very sticky and reduced with mineral spirits. Use a plastic spoon and mineral spirits to tool and smooth the sealant.

Caulking windows and doors is the same on a concrete block building as with caulking any interior or exterior surface.

Choosing the Primer and Paint

Primer

Unpainted concrete block needs to be primed before the application of the finish paint. The main function of the primer is to fill the pores and neutralize the high P.H. The best primer designed for this purpose is Acrylic Block Fill.

Typical coverage for Block Fill is 200 square feet per gallon on smooth block and 100 square feet on split-face block.

Acrylic Block Fill primer can be applied over acrylic or latex painted concrete block when properly cleaned and prepared. Many manufactures also produce an oil base formula for use over oil base finishes. This can resolve prior defects in paint jobs that were never properly primed and have open pores.

Paint

Any high quality paint can be applied over Block Fill primer, but certain types perform better over time. The absolute best for exterior use is Elastomeric paint. This finish can outlast normal paint 2—1. The next best choice is 100% Acrylic semi-gloss or satin sheen paint. Interior surfaces can receive any sheen to match your decor.

Acrylic paints will out last standard oil base finishes when use on the exterior. With interiors your paint choice depends on the use of the wall, room or area. Finishes with a higher sheen, semi-gloss or satin, will be easier to clean and maintain.

Applying the Primer and Paint

In most cases the primer and paint should be applied to concrete block with an airless sprayer and back rolled. This is a two-person operation.

Block Fill primer is thick and will require a powerful airless sprayer. A 3/4 gallon per minute sprayer and a 517 spray tip is the minimum required. Large professional units can be rented. Use a gas powdered sprayer on the exterior for added power and convenience.

The roller cover needs to be a thick nap and of the highest quality. Cheap synthetic roller covers become compressed easily and will be frustrating to work with. 100% lambs wool covers work best. Use 3/4—1 inch for smooth block and 1-1/2 inch on split-face block.

Apply a thick even coat and immediately use the roller to push the primer and finish paint into the pores of the concrete block. The goal is to produce a surface that is completely filled and sealed against moisture. This procedure works equally well on interior block surfaces. Back rolling is a great way to produce a better looking result even without the need for primer.

Spraying isn’t always necessary, standard brush and rolling techniques can be used. Dipping and rolling will be slower and more labor intensive, but when painting the interior of occupied block buildings and homes rolling can be the best application method.