Latex Paints

Moisture

The porosity of paints varies with the pigment/resin ratio of the product. Gloss paints have a lower pigment content than flat paints and are less permeable than flats and more of a vapor barrier. The resin used also determines the vapor characteristics. Moisture vapor transmission is rated in "perms" and some typical perm ratings are listed below: A perm rating of less than 1.0 is considered a vapor barrier.

2 mil polyethylene (medium density)   0.4

Epoxy-polyamide (gloss)                     0.14

Alkyd semigloss                                   0.57

Latex semigloss                                   4.98

Alkyd flat                                               19.9

Latex flat                                               27.0

Don’t Use As A Primer...

Never use a flat latex paint as a primer under eggshell paints. Flat paints have a high pigment to resin ratio and are very porous. When an eggshell product is applied over a porous surface, some of the resin is absorbed into the flat which causes a variation in sheen (flashing) in the eggshell. Usually when the surface has been rolled, the gloss on the top of the stipple (where the paint is thicker) is higher than the bottom of the stipple, giving the surface a mottled look.

Acrylic Latex

Acrylic latex paints have better "wet adhesion" than PVA latex paints. Wet adhesion, simply stated, is the adhesion of a product to a previously painted surface when it becomes wet. Some latex paints adhere well in dry conditions, but when washed, or exposed to condensation (like in small unvented bathrooms with shower), the product washes off or can be peeled from the surface to which it was applied.