...really silly question I think, but i'm going to ask for you all to chortle at.
i have been preparing a spare set of cups for refinishing, yet after 3 days the primer still hasn't dried in places I am assuming that this is in direct relation to the areas where I've not quite gone down to bare metal in removing the original paint?
in essence - does etching primer need raw metal to bit into?
thanks in advance...

if i remember right etch primer was used on aluminium

Don't think etch primer 'needs' bare metal but probably better to remove all the old stuff, soft spots i guess would be down to either too not letting it dry properly between coats and putting it on too thick. Or possibly not shaking the can for long enough. Not that i'm in any way a professional but i have learnt from the many mistakes i have made over the years.

Etch should only be used on metal , it has acids which as name says etch in to the metal for adhesion. Once dry the acid has done its job and offers very little corrosion properties . These acids can also soften existing paint around the repair.

Etch should be applied in one lite coat , thats all that is needed. Multiple coats of an acid etch primer will add nothing .

Etch is a adhesion promoter , if your looking to seal the metal then your using the wrong product. Epoxy is far superior at sealing the metal and protecting against corrosion .