If you need thicker house paint, you can always simply remove lid from the can and leave it open and let some of the solvent evaporate. That is probably your best option if the paint is oil-based; if the paint is latex, you can buy an additive that does the job more quickly. A suitable thickening product increases the paint viscosity without affecting other properties of the paint, including adhesion and color. Corn starch and flour may work, but avoid them; they promote mold growth. A better choice is hydroxyethylcellulose, which is a common ingredient in certain cosmetics.
What Is Hydroxyethylcellulose?
Derived from the cellulose in wood, HEC is a white powder that is used for thickening hand creams, hair conditioners and lotions. It readily dissolves in hot and cold water to create a slightly acidic solution that feels smooth to the skin -- although you probably don't care about how it feels when you're thickening paint. Because of its usefulness in the cosmetic industry, you might have more luck finding HEC powder in a cosmetics store than a paint store. You can also buy HEC products specifically designed for thickening paint.
Thickening Paint With HEC
The main consideration when mixing HEC with latex paint is that adding the powder to the paint too quickly results in lumps. You can avoid this by premixing the HEC with water to create a gel and adding the gel to the paint. This method produces a stock solution of gel that you can use to thicken several gallons of paint.
Pour a measured quantity of water into a bucket or -- preferably -- an electric mixer. You may choose to mix a quart or a gallon of thickener, depending on much paint you have.
Start the mixer; if you don't have a mixer, begin stirring the water with a stir stick or a hand-held rotary mixer. You'll probably find it helpful to have a second person to help.
Sift the HEC powder into the water as you continue stirring. The viscosity of the water will change as you add the powder; stop when the mixture becomes thick and gel-like.
Add a preservative to the mixture to prevent deterioration if you plan to store it for future use. A good candidate is sodium benzoate, a food and cosmetic preservative. You don't need much -- 0.1 to 1 percent is sufficient. If you have a quart of thickening agent, a teaspoon of sodium benzoate is all you need.
Add the thickening agent to the paint, stirring as you do, until the paint is thick enough for your purposes.