Gloss and flat paints vary in their concentration of pigment relative to binder. The pigment is the ingredient that gives the paint its color, whereas the binder is the glue that holds the paint together and to the surface you're painting. Binders are resinous and reflect light, while pigments absorb light. You can make your gloss paint flat by adding more pigment, but because pigments are porous and nondurable, you'll be reducing the washability of the surface.
Pigment Volume Concentration
The sheen of a paint product -- which is a measure of the amount of light it reflects -- is determined by its pigment volume concentration -- or ratio of pigment to other ingredients. The higher the PVC, the flatter the paint; products labeled flat -- as opposed to semigloss or satin -- typically have a PVC of 40 percent. Gloss paint, on the other hand, has a PVC of roughly 15 percent. Consequently, if you want to change a can of paint from gloss to flat, you have to add enough pigment to increase its volume by roughly a quarter. This means that, if you have a gallon of gloss paint, you need to add approximately a quart of pigment to make it flat.
Alternatives to Adding Pigment
You might find adding such a large volume of pigment to your paint impractical for at least two reasons. One is that it will likely be difficult to maintain the color, and another is that the paint won't perform as well -- it will rub off more easily and generally be less durable. A couple of simple alternatives might eliminate the need to flatten the paint mixture.
Topcoat With Flat Varnish
Manufacturers use light-dispersing additives to make gloss varnish flat, and these additives will also flatten a gloss paint finish. Use your gloss paint as it is, applying one or two coats as necessary, then apply a topcoat of flat varnish. You can buy varnish products specifically made for this purpose.
Degloss the Finish
After gloss paint is dried, you can degloss it in one of several ways. One way is to wash it with a strong mixture of trisodium phosphate and water:
- Mix a solution consisting of 1 cup of TSP per gallon of water in a bucket.
- Wash the glossy surface, using a sponge. Be sure to wear rubber gloves and goggles.
- Rinse with clear water.
In lieu of TSP, use a commercial deglossing agent or, if the paint is oil-based, rub it down with denatured alcohol. Don't use alcohol on latex paint, though -- it will remove some of the color.
You can also degloss the finish using 220-grit sandpaper. Scuff lightly by hand -- you don't need a machine for this -- and wipe off the sanding dust with a damp rag.