Buy a thickening agent. You can buy a thickening agent for your paint from a local home improvement store. Most thickening agents for latex paint are made with water-soluble hydroxyethyl cellulose which works well with the latex.
Make sure the thickening agent says it works with latex paint.
Add the thickening agent to the paint. Read the directions on the bottle of thickening agent to find out how much to use. Typically you will add it by the tablespoon depending on how much paint you have.
For the best result, add a smaller amount than called for, and then slowly add more until you get the thickness you need.
Adding more than the manufacturer’s recommended amount can cause the paint to crack and peel when applied to the wall.
Stir the paint. Use a paint stick to slowly stir the thickening agent into the paint. The paint will thicken as you stir. If the paint still isn’t thick enough, add more thickening agent in small amounts as you stir.
Test the paint. Paint a small part of the wall to test the thickness of the paint. Let this paint dry completely before checking the results. The paint should not have cracks or be peeling in any way. If the paint looks great and has an even color, you can finish painting the rest of the wall.