Car paint should always be applied to a fully prepared smooth surface and preferably in warm weather as paint takes longer to dry in cold conditions. Wash and dry the vehicle and then use fine wet and dry sand paper (2000 grit) to rub down the area you wish to paint. This creates a “key” for the new for the new paint to adhere to the surface. If there are any deep scratches , you might need to use a Simoniz primer or Spray Putty to fill them before applying the colour coat. Use Simoniz Colour Restorer Polish to blend new paintwork into old and to remove any fine scratches or swirls.