If you apply thin semigloss paint to a vertical surface, excessive running and sagging could result. Fortunately, you can discourage this sort of problem by adding a thickening agent to the paint. Unfortunately, if you add the wrong type of agent, you'll likely ruin the finish. Many inexperienced novices recommend using anything from flour to baby powder to thicken semigloss paint. Avoid these sorts of home remedies and opt instead for a particular type of thickening agent that won't cause streaking or discolouration.
Pour 2 gallons of semigloss latex paint into a 5-gallon bucket.
Add a cup of hydroxyethyl cellulose to the paint.
Affix a 20- to 30-inch-long mixing paddle to your drill. Submerge the head within the semigloss paint. Start the drill and keep it running for at least three minutes.
Turn off the drill before removing it from the thickened semigloss paint.
Semigloss paint contains chemicals that provide it with a high level of sheen. Hydroxyethyl cellulose will thicken the paint without affecting the sheen. Other thickening agents may cause the sheen to dry unevenly. Don't try to mix hydroxyethyl cellulose with semigloss paint, using a wooden stir stick, or the sheen may dry unevenly.