Proper gun cleaning is an absolute must and should be done immediately after spraying.  

The cup, air cap, fluid tip and needle should be removed and all parts should be cleaned using lacquer thinner and the proper brushes and rods to clean the paint from ALL the inner and outer surfaces.  

This kit gives you the tools necessary to do the job properly.

Inlet bulb filters can be the last line of defense for air 
that can be less than perfectly clean.  It's hard enough to keep dust and other contaminants out of your paint job, you don't want to be blowing dirt in from your compressor.

The needle and air control pin should be lubricated in order to keep them working smoothly and not sticking during the spraying process.