Clean the car thoroughly, including machine mopping or claying if necessary. Dry the car and then use a Quik Detailer and wipe over the surface again, removing any dust, watermarks, etc... from the paintwork. Sealers do EXACTLY what they are marketed as. They seal paint and anything on it, including watermarks.

You can then either use a filler polish (something like AG SRP) to mask any fine scratches or, if the paintwork is to a suitable standard, go straight in with the AG EGP. If using a polish first, remember to redo a QD once-over once cured to remove any polish residue or dust.

Apply the EGP gently to an applicator pad and wipe over the surface, ensuring even coverage. Trust me, a little goes a long way with sealers.

Leave it for @ least 30 minutes to an hour to cure properly, and do not apply to hot paint or in direct sunlight. Remove with a microfibre cloth and then apply a decent wax after another application of QD.