How To: Prepare Concrete Before Painting
Before painting a concrete floor, the floor should be tested to see what preparation is needed to assure the coating will adhere well.
Test for the presence of a curing compound
Most concrete floors had a chemical curing agent applied after the concrete set up. Many of these curing agents will prevent good adhesion of coatings. To test for a curing agent, pour a little muriatic acid on the floor. If the solution bubbles, there isn’t any curing compound left on the surface or there wasn’t one to begin with. If the acid doesn’t bubble, then there is a curing compound on the surface that must be removed before painting. Check several different areas of the floor because the curing compounds may have worn off unevenly.
Removing curing compounds
Curing compounds must be removed with either chemical removers or mechanically by floor machines equipped with screeds or shot blasting.
Test for grease and oil
Coatings will not adhere to grease and oil. Unfortunately, grease and oils are commonly found on concrete floors. Dark spots are an indicator that grease and oil are in the floor. Sprinkle a little water on the floor. If it beads up, grease or/and oil may be present. Note that a curing agent may also bead up water. Another test is to apply a piece of duct tape to the area in question. If the tape pulls away very easily, grease or/ and oil are present. Grease and oil must be removed before coating.
Removing grease and oil
Apply granular oil dry compound to the dark spots and areas where it is obvious that oil and grease are present. Let stand overnight. Sweep or vacuum the compound up. Apply a good commercial oil and grease remover and follow the manufacturers recommendations. Use an electric floor scrubber or a stiff bristle brush. Alternatively, use a strong TSP solution and floor scrubber or bristle brush. Rinse well and allow to dry. The floor color should be relatively uniform. If there are dark spots, put a little muriatic acid on the spots. If the acid bubbles, the floor is probably ok to coat if the acid doesn’t bubble, grease and/ or oil are still present and the floor must be degreased again. Repeat these steps until the floor is completely free of grease and oil.
Testing for adhesion of existing coatings
Existing coatings do not necessarily have to be removed. It may be possible to coat right over them if they are in sound condition and are adhering well to the concrete. To test adhesion, score the existing coating down to the concrete with a shar knife. Make five cuts about two inches in length parallel to another about ¼” apart. Make five more cuts at 90 degrees to the first so you end up with 50 little ¼ inch squares. Apply duct tape to the area and firmly press it in place. Pull off the tape. If the coating doesn’t pull up the little squares, the coating is ok to leave in place. If a significant number of squares come up, the coating needs to be removed. It is normal to get a little dry paint on the tape where the cuts were made. This is ok.
Removing old paint
Abrasive blasting with shot, beads or sand can be used to remove old coatings. Chemical removers can also be used by following the manufacturers instructions.
Acid etching assures the concrete provides a good surface for the coating to adhere. After the floor is clean and degreased, apply a 50-50 mixture of muriatic acid (20% strength) to the floor. CAUTION: MURIATIC ACID CAN CAUSE BAD BURNS. WEAR GOGGLES, PROTECTIVE CLOTHING, RUBBER GLOVES AND RUBBER BOOTS. ALWAYS ADD ACID TO WATER, NEVER ADD WATER TO ACID. Allow to remain on floor for 15 minutes. The acid will bubble up as it reacts with the concrete. Rinse with water. Rinse again with water and ammonia to neutralize the acid. Rinse a third time with plain water. After the surface is dry, it should feel like medium grit sandpaper. If the concrete is still smooth, a second application of muriatic acid is needed.
The floor is now ready for coating. Follow the paint manufacturers instructions for coating. We recommend Ultra Tred 2 part epoxy or Aqua Borne Ceramic Floor Enamel.