The 400 grit can be used to remove mould seams scratches traces of glue putty and other defects. With the 4000 you will be able to polish the surface. This side is especially useful for polishing clear plastic parts removing the glossy finish from decals or polishing previously painted surfaces. The flexibility of the material easily conforms to any shape including rounded surfaces. Can be used on a full range of materials including resin metal wood and of course plastic. It can be used both dry and moistened with water.