How do I prepare a concrete floor before painting it?

Coating a garage floor is one of those jobs where the visible part is a fraction of the work, and where the temptation to skip ahead is strongest because the floor already looks fine. It is worth being blunt about this: preparation is the job. The rolling on is an afternoon at the end of it.

Safety

Step by step

1. Test for moisture and take the result seriously.Tape plastic sheet down for a couple of days in several places. Moisture underneath means the slab is passing water, and virtually every coating will eventually blister off it. That is a stop-and-reconsider result, not a delay.
2. Deal with oil contamination completely.Oil soaks in and stays. Degreasing the surface removes what is on top; oil that has penetrated needs drawing out or the affected area cutting out. Coating over it produces a permanent bare patch.
3. Remove any existing coating or curing compound.New coatings bond to the old one, and the old one may not be bonded to much. Curing compounds left from construction are invisible and are a common cause of total adhesion failure.
4. Open the surface mechanically.Diamond grinding or shot blasting gives a consistent profile and removes weak surface material at the same time. This is the reliable method and it is what the product data sheets assume.
5. Use acid etching only if you must, and understand its limits.It roughens a surface but does not remove contamination, does not work on a dense power-floated slab and leaves residues that must be neutralised and rinsed thoroughly. It is the fallback, not the default.
6. Repair defects after preparing, not before.Grinding reveals pits, cracks and weak areas that were not visible. Repair them once you can see them, using a product compatible with the coating.
7. Vacuum thoroughly, twice.Dust is a bond breaker. Use a vacuum suited to fine dust, then go over it again. A tack pass afterwards is not excessive on a floor.
8. Check the temperature and the forecast.Both the slab and the air need to be within the product's range, and the slab lags the air by a long way. A cold slab in an unheated garage is the commonest reason a coating stays soft.
9. Do the water drop test before opening the tin.Flick water onto several areas. It should soak in within seconds. Where it beads, the surface is still sealed or contaminated and the coating will not bond there.

Common mistakes

• Skipping the moisture test because the floor looks dry. — A slab passing moisture looks completely dry on the surface, because the water evaporates as fast as it arrives. It stops evaporating once you put a film over it.
• Acid etching a power-floated slab and calling it prepared. — Dense burnished surfaces barely respond to acid. The floor looks etched and is still too closed for the coating to key into.
• Painting over an oil patch after degreasing the surface. — The oil is in the concrete, not on it. It migrates back out through the coating and lifts it in exactly that spot.

Questions people ask