Daddyman wrote: He then says the "real" primer is etching primer, "which has an important job to do on non-ferrous metals. After application, an acid in the mix, over a period of 12-24 hours, eats into the metal surface, creating the microscopic roughness that the paint requires to improve adhesion." (This acid, he says later, is our old friend phosphoric acid!) Two top tips for etching primer (as opposed to car primer) are (1) this thing about leaving it for 24 hours (for the Precision two-part primer) (don't touch it at all during this time - who knows what the blend of finger-print grease and, these days, alcohol-rich hand sanitiser, will do to mess up the process); and (2) it can (should?) be thin enough that it doesn't obscure the different colours (brass, whitemetal, resin, solder) beneath;
And to illustrate that point, these are a couple of my E22 having just been primed, and before going under the dust cover to harden off. The colours of the base metals underneath are clearly visible. This is with Comet self-etch primer, IIRC:
The nickel silver coupling rods are being primed as they are painted vermilion on the prototype.