Hey! Somebody get the door
The first photo shows a bare metal door. To ensure that all subsequent layers adhere properly, the metal is treated with an acid etching primer that bonds to the metal and provides a surface that the primers can stick to. The etching primer can be seen as the golden color in the second photograph.
After the etching primer comes the epoxy primer, seen as black in the third photo. Epoxy is a glue, and this primer is designed to stick to the etching primer, seal the car against moisture to prevent rust, and to provide a bonding surface for the next layer.
The last two photos show the high builds primer. The high builds primer function is to fill and smooth any small imperfections so the paint is laid on a perfectly smooth surface to give that ultra high gloss look that everyone likes so well. Most of the high build primer will be sanded away leaving only small amounts here and there as filler for the waves and ripples in the car’s metal.