I am assuming this is a latex based paint? You can call the Dow company and ask them if they sell hydroxyethyl cellulose (Cellulosize ER 15) to the public. It is the only thing I know of that can be used to thicken latex paint. DO NOT use flour or starch, as these things can lead to molding within the paint. Dow's US number is 1-800-447-4369. You may also call Lowe's or Home Depo to see if they sell any paint thickening products. The only other recourse is to take the lid off, but it may cure, and thus make remixing difficult. Good luck. Susu

