Superficially simple, but is it ? Before I start I have been trying to envisage the various spray angles from an aerosol but reaching the inner surfaces of the coil could be tricky without excessive overspray/sags on the outside. Any tips?
Somewhat related, can you use POR15 chassis black over 'conventional' paints. I was thinking this might be ideal stuff for the job especially as I have an opened mini can that I might otherwise have to throw away. I was thinking that a sort of long 'toothbrush' type brush would deal with the inner coils issue above. Any advice appreciated.

Pretty sure paint won't last long on a spring.

if using powdercoat, its usual to electrically charge the powder, so its attracted to the work, which works well for difficult to access positions. I believe it can be used for paint too

I spray painted my Cobra springs with Duco rattle cans, the overspray when painting the spring painted the back of the spring plus a spritz down the centre. It lasted some twenty years of light use (I would guess at 40,000 to 60,000 kms - never connected up the speedo so don't know how far I drove it.)

I would avoid thick paint, so a light coat of spray and leave it at that. Ideally etch first then topcoat.
If you want to use the por15, it should be on bare metal, and as above, thick coat means likely to crack.

If you do want to brush it on, use an angled radiator brush?