Primers & Undercoats – stock Primers & Undercoats variations. This category lists all of the primers and undercoats that we hold in stock. All of these are used underneath the topcoat. The Etch Primers are essential when painting the vehicle or boat from the bare metal. The Etch primer will stick to the metal unlike the filler primer or undercoat. The filler primer and undercoat dims the impurities from the previous etch primer to have a smooth surface to paint on. This prevents the end product appearing patchy or unsmooth looking.