The Trade 20/Trade 4 contain a specially formulated solvent (Brush mate Fluid) which fills the product with a vapour. This vapour prevents the paint on the brushes from drying. It does this by stopping the paint solvents 'flashing' off the brush and also by preventing the chemical drying process (polymerisation) that normally takes place as paint dries. In the Trade 20 the solvent (Brush Mate Fluid) is drawn from a bottle and up a wick onto the central vaporiser panel. The Brush Mate Fluid evaporates from the vaporiser panel to create the vapour. Instead of a bottle of fluid and central vaporiser panel, the Trade 4 contains a panel (Vapour Mate) which is impregnated with a solvent similar to Brush Mate Fluid.