● Concrete and moisture

Epoxy primer. One coat or two?

11 Nov 20261 min readFORSPEC Technical Team

The short version

One coat of ACTFLEX EP250 primes and seals the concrete: 150 µm dry.
Two coats make a moisture barrier: 300 µm dry.
Two coats are needed where the slab reads over 4.5%.

A primer does two jobs. It gives the membrane something sound to stick to, and on a damp slab it holds back the moisture that would push the membrane off. The second job takes more film.

The figures

Coverage: 0.32 kg/m² per coat
Dry film: 150 µm per coat, 300 µm in two
A 20 kg kit: About 62.5 m² in one coat, 31.3 m² in two
Recoat: 4 to 24 hours
Membrane after: 24 hours at least
Pot life: About 90 minutes at 25°C

When one coat is enough

On sound, prepared concrete that reads 4.5% or under, one coat seals the surface and primes it for a FORSPEC membrane.

When it has to be two

The slab reads over 4.5% on a meter.
Relative humidity in the slab is over 80%, or vapour emission is over 15 g/m² per 24 hours.
The job is moisture control: a damp slab, rising damp or efflorescence.

The first coat may be thinned with clean water on porous or dense concrete. The second coat is never thinned, and it goes on 4 to 24 hours after the first.

Mixing

Part A to Part B, 1 to 1 by volume, never by weight. The kits are pre-measured, so mixing the full contents of both is right. Mix by drill for 2 to 3 minutes.