Preparation - Concrete Floors

New Concrete

Before recommending a coating for a new concrete floor, it is important to know if curing membranes or concrete hardeners were used by the floor finisher. Paint does not stick well to these materials and failures usually result when they are painted over. If water beads on the surface of the floor, or acid* etching doesn't seem to work, it is likely that a membrane or hardener was used. Membranes can be removed with paint remover, but, hardeners must be mechanically removed (abrasive blasting by Blastrac™).

Muriatic Acid*

When mixing muriatic acid* with water, always add the acid* to water — never water to acid*. A lot of heat is given off when the two are mixed and this may cause foaming and/or splashing if done the wrong way around. The proper solution for acid* etching concrete is 10%. Since muriatic acid* is only 38% acid*, that means 1 part acid* to 3 parts water — not 1 part acid* to 9 parts water.

Etching

The best way to acid etch a floor with muriatic acid* is with a plastic watering can, depositing the acid* on the floor in two directions to assure coverage. Allow the acid* to work 30 minutes (or until the fizzing stops) and then flush with water to remove the salts formed by the acid* reacting with the concrete. Using a mop to apply the acid* does not give a consistent etch.

When To Paint New Concrete

Concrete floors should not be painted for at least 60 days after installation. It takes 30 days for the concrete to cure and a further 30 days for the balance of the moisture to leave the concrete.

Laitence

Concrete floors that have been finished with a mechanical float (trowel), usually end up with a scum on the surface called laitence. Laitence is a mixture of cement and sand that is loosely bound to the cement floor. It can be removed with a mixture of 1 part muriatic acid* and 3 parts water. The acid* mixture is best applied to the floor using plastic watering cans. Allow the acid* to work about 30 minutes before rinsing the floor with water. It will take at least 72 hours for the floor to be dry enough to paint. It is laitence that causes a concrete floor to be dusty. Many paint failures on floors have laitence stuck to the back of the peeling paint.

Armored Floors

Etching a concrete floor with muriatic acid* can cause armored floors(floors containing steel filings) to rust. It can also cause corrosion of electrical contacts in areas where there is a lot of electrical equipment. In both cases the problems can be reduced by using phosphoric acid* in place of muriatic acid. A 10% solution of acid* is recommended.

Testing For Moisture

Before painting a new concrete floor, it is important to know if the surface is dry. New floors take about 60 days to dry under good conditions. To test for moisture, lay a 2' x 2' piece of 4 mil plastic on the floor and seal the edges with duct tape. If no condensation appears under the plastic after 48 hours the floor is likely dry enough to paint. Grade and below grade floors without a proper vapor barrier may continue to transport moisture and never hold paint properly. For more accurate indications of moisture, an electronic moisture meter should be used.

*Important Note: Always Wear Rubber Gloves And Goggles When Handling Acid.