Concrete Primer:
Which Type, When You Need It
& How to Apply It

Types of concrete primer compared

Acrylic masonry primer-sealer

For walls, foundations and other above-grade masonry under latex paint. Sherwin-Williams Loxon Concrete & Masonry Primer is a common example: 200–320 sq ft per gallon, 2.1–3.2 dry mils, goes on concrete at least 7 days old with a pH from 6 to 13, minimum 40°F, 24 hours to recoat. It is not a floor primer.

Block filler

A high-build primer for concrete block, which is too porous and pitted for paint to cover alone. Behr Pro Block Filler covers 75–100 sq ft per gallon, wants masonry cured at least 30 days with moisture under 15%, and is for above-grade walls only. Back-roll it into the pores; do not just spray and walk away.

Bonding primers for self-levelers and overlays

These are thin, water-based products that stop a cement underlayment from losing its water into the slab and help it bond. Ardex P 51 is mixed 1 part primer to 1 part water on normal absorbent concrete, or 1:3 then 1:1 on extremely absorbent concrete, covers 400–600 sq ft per gallon, and needs 30 minutes to 24 hours before the underlayment goes on. Always use the primer the underlayment maker specifies.

Penetrating epoxy primers

Used under epoxy, polyaspartic and many polyurea floor systems. They are thin so they soak in. MiraPrime ML, a water-based 2-part epoxy, covers 320 sq ft per gallon at 5 wet mils and is designed to penetrate up to 300 microns into concrete, with a topcoat window that depends on what goes over it.

Moisture-vapor-barrier epoxy primers

Heavier epoxy primers for slabs that test too wet for a normal system. MV2112, as an example, is rated to control vapor emission up to 20 lb per 1,000 sq ft per 24 hours (ASTM F1869) and 75–95% RH (ASTM F2170), applied at 17–18 mils, about 90–94 sq ft per gallon, over at least a CSP 3 ground profile. Tnemec, by comparison, puts the general limit for most primers around 3 lb or 75–80% RH.

Polyurethane and polyurea primers

Spray polyurea systems have their own primers chosen for the substrate. Our ArmorPrime line includes UltraPrime, a fast-cure polyurethane primer, WC-111, a modified polyurethane designed to react with moisture and contaminants for adhesion, and Primer 41, a two-component multi-purpose primer. Which one goes on a given slab is set by the system specification, not by guesswork.

Concrete primer selector: which primer, or none?

Tell us what goes on top and what the concrete is like. The selector picks the primer category, flags age and moisture problems, and tells you when the topcoat's own data sheet should decide.

How to prime concrete the right way

1. Read the topcoat data sheet first

   The topcoat maker names the primer, the profile and the recoat window. Mixing brands across one system is the fastest way to lose any warranty and the second fastest way to get delamination.

2. Profile the surface

   Primers only bond to clean, open concrete. For paint and thin primers, an etched or lightly ground surface is enough. Thick floor systems need grinding or shot blasting; the MV2112 vapor-barrier primer, for example, calls for at least ICRI CSP 3, which feels like 60-grit sandpaper.

3. Remove all dust

   Vacuum, do not just sweep. Grinding dust left on the slab becomes a weak layer between primer and concrete.

4. Test moisture

   Plastic sheet for paint projects; calcium chloride (ASTM F1869) or in-slab RH probes (ASTM F2170) for coating systems. The readings decide whether a standard primer will do or a vapor-barrier primer is needed.

5. Apply thin and work it in

   Most primers fail from being too thick, not too thin. Roll or squeegee, then back-roll so it soaks in. Avoid puddles in low spots. Block filler is the exception: it should be worked into every pore.

6. Hit the recoat window

   Topcoat inside the window the data sheet gives. Too soon and solvents or water get trapped; too late and some epoxy primers cure too hard for the next coat to bite, which means scuffing or re-priming.

Where our primers fit, and where they do not

ArmorPrime products are made for our polyurea and polyurethane systems. If you are painting a basement wall or a porch, buy the primer your paint maker specifies; our primers are not a substitute and would be overkill. Where we do fit is a coated floor, a tank, a containment area or a concrete coating project where the primer, membrane and topcoat are specified together. There, the applicator chooses the primer based on the surface and the moisture readings, and the whole stack is designed to bond.

For more on preparing concrete for any coating, see our surface preparation article, the garage floor coating guide and the epoxy floor coating guide.