To paint or not to paint. Concrete and paints are two different kinds of animal. Fortunately paint bonds readily to other materials, including concrete. The physical condition of the concrete, its moisture content, alkalinity and porosity, as well as the cleanliness and integrity of the surface, all affect the success of an application. For most coatings the surface should have the texture of medium grade sandpaper. However, some heavy-duty epoxy surfaces for flatwork may require a broom finish to be produced when the concrete is being constructed to obtain maximum adhesion. Other materials or surface conditions that may make trouble for successful coating of concrete are: laitance; efflorescence; smooth, hard-troweled surfaces; liquid hardeners; form release agents and curing compounds; bugholes and honeycombing; and fins and protrusions. Laitance is a layer of weak and nondurable material containing cement and fines from aggregates, brought by bleeding water to the top of overwet concrete, the amount of which is generally increased by overworking or over manipulating concrete. Because laitance has poor bond and strength, any coating applied over ti will eventually peel. For proper coating performance laitance must be removed by blasting or acid etching. Contractors sometimes forget that a protective coating is to be applied after construction. Form release agents containing large amounts of waxes or soaps often are used because they strip easily. To some extent, these compounds remain on the surface where they have the same effect on coatings as on forms: they won't permit coatings to adhere. Acid etching won't remove these materials, and a drastic measure such as sandblasting may be needed. Liquid hardeners for concrete surfaces are sometimes applied to correct deficient hardness or decrease permeability to liquid and provide resistance to mild chemical attack. Most commonly, sodium silicate or the fluosilicate of magnesium or zinc is used. They should not be used when coatings are to be applied. Although they may make concrete hard enough so that it no longer dusts and can't be scratched with a coin, organic coatings will not adhere to hardeners. Acid etching will also not remove hardeners. The only way to prepare a hardened concrete surface properly to receive paint is to sandblast the surface.