A mathematical relationship between the pigment contained in a particular paint product to the binder in that product determines the sheen. To state this relationship -- known as the pigment volume concentration -- simply, you would say: "The more pigment the paint contains, the flatter it will look on your wall." If you have flat paint, and you want to make it glossy, you can do it by adding more binder. If you've already applied the paint, and it's dry, you can make it glossy by painting over it with a clear gloss finish.
Pigment Volume Concentration
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The formula for pigment volume concentration, which is expressed as a percentage, is simple:
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PVC = Pigment volume/pigment volume + binder volume X 100
Flat paints are generally those with a large PVC, while the PVC of gloss paints is small. Semigloss, satin and eggshell sheens are between these extremes:
- Flat: PVC is 38 percent to 80 percent
- Eggshell: PVC is 35 percent to 45 percent
- Satin: PVC is 35 percent
- Semigloss: PVC is 25 percent
- Gloss: PVC is 15 percent
It's a fairly simple matter to flatten glossy paint -- you just add pigments. It isn't quite as straightforward to turn a flat paint into a glossy one because to do so, you have to add compatible binders, and binder compatibility is more dependent on the paint chemistry than pigment compatibility. When you add binder to paint, you're essentially creating a glaze, which is something that faux finishers do regularly.
From Glaze to Gloss Paint
A typical can of paint contains between 30 percent and 45 percent pigments and binders -- the rest is additives, such as thickeners, surfactants and solvents. If the paint is flat, it means that the can contains anywhere from 12 percent to 35 percent pigments, and you want to reduce that to something more like 5 percent to 15 percent; in other words, you want to increase the volume of the paint by 7 percent to 20 percent by adding binder.
Faux finishers make glaze by adding paint to a compatible binder. In the case of latex paint, a compatible binder might be commercial glazing liquid, poly-acrylic varnish or even waterborne polyurethane varnish, depending on the paint. For oil paint, use an oil-based glazing compound. A test is definitely in order before you mix an entire batch.
To make a gloss paint out of a flat one, pour an amount of glazing compound or varnish equal to about 10 percent of the volume of flat paint you have into a large container, add the paint and stir. Test the result and add more binder if the sheen isn't glossy enough. Do it incrementally, testing as you go, to avoid overdoing it and turning your paint into a semitransparent glaze.
Glossing Up a Flat Finish
If you used flat paint on a wall or piece of furniture, and you're dismayed by how easily the paint scuffs and bruises -- which is a result of the large amount of pigment it contains -- it's easy to give it a glossy face lift. Simply paint over it with a clear, glossy varnish. Polyacrylic varnish is a good choice for most surfaces, whether covered with latex or oil paint. You can also use wallpaper varnish -- a thinner water-soluble product that comes in gloss, satin and flat.
Mixing Flat and Gloss Paint
If you're wondering about making your flat paint glossy by mixing it with more gloss paint, keep in mind that you have to reduce the PVC in the flat paint by 25 percent or more. This approach might work if you add a quart of flat paint to a gallon of gloss paint or a gallon of flat paint to a 5-gallon bucket of gloss paint, but you'll end up with satin or semigloss if you mix equal quantities of gloss and flat paint.