The quality of the paint job depends on a perfectly clean surface. The surface may not need sanding and peeling, but it must be clean. Trisodium phosphate is a kind of powerful degreaser, which is always be used to remove dirt, grease, mold and soap residue from the inside and on the surface. This chemical will degrease a shiny surface so that the next layer of paint will stick. TSP chemical can also be used to clean and soften hardened paint brushes.