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 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.
One of the weaknesses of alkyd paints is the tendency to yellow with age. Yellowing is accelerated in the absence of light (e.g. behind pictures) or if the dry paint is exposed to fumes from ammonia or the amines in epoxy paints. On new construction, damage to surfaces painted with alkyd paints can be caused by fumes during the application of epoxy flooring products, or from ammonia in cleaning products. If yellowing occurs, repainting is necessary.
Alkyd paints that contain substantial amounts of linseed oil (some house paints and tank enamels) are prone to wrinkling when used at low temperatures. These products require 48 hours dry time at 77ºF (25ºC) before recoating. If these products are topcoated before they have dried sufficiently, wrinkling will occur. If the surface is cold and the humidity is high, dry time is affected. A fast-drying topcoat applied over an undried or soft primer will also wrinkle.