Step 1
Pour a measured cup of paint into a container that can hold at least 2 cups.
Step 2
Add 1 tablespoon of paint thinner to the cup of paint.
Step 3
Stir the paint with a paint stick. Scrape the bottom and sides of the container to make sure you mix all of the paint.
Step 4
Paint a thin streak of the thinned paint onto a piece of scrap paper or cardboard to test its thickness. Wait a few minutes for it to dry to ascertain true coverage and color.
Step 5
Repeat Steps 2 through 4 until you reach the desired consistency. Write down the number of tablepsoons of paint thinner required.
Step 6
Calculate how much thinner you need to thin the main batch. Most paint comes in quarts or gallons. There are 4 cups in a quart and 16 cups in a gallon. Multiply the number of tablespoons you added by the number of cups remaining in the container (minus the cup you already took out). There are 16 tablespoons in a cup.