Paint not sticking to metal

[FarmerPug]

A general question about painting metal, recently i ground the 305 driveshafts and anti roll bar back to bare metal getting rid of the flaky rust, they were nice and shiny, then sprayed on 3 coats of zinc 182 which claims to be anti rust primer, each coat was given a few hours to dry, then a day after the final coat, and finally painted with spray stone chip shield and also let dry, they have been dry for a few weeks now but i recently set one of the driveshafts down and scuffed it, but the paint went right back to the bare metal. What have i done wrong?

[MixMan]

At work I paint a lot of aluminium and to prepare for painting I use a two part etch primer. If I dont then the paint scratches off with your finger nail. Not sure if steel needs it as I never paint steel, but, just a suggestion.

[binnzy]

did you degrease it first ie wash it off with thinners any grease or oil and the paint wont stick even getting hold of it with dirty hands will contaminate the metal and the paint wont stick