Filler Primers – stock filler primer variations. This category lists all colour variations for the filler primers and related products. Filler primers are used to fill in imperfections in a surface. The filler primer prevents the finished top coat from looking unsmooth and having uneven surfaces. Coach Enamel primer generally contains a rust inhibiting element, it fills pores in the bare metal and protects bare ferrous metal surfaces. It is manufactured from (in paint terms) a fairly coarse material and cannot be prepared to the same standard as undercoat.