I got a price for spraying the tanks from a company recommended as being reasonably priced: £200 per tank! each one nearly twice as much as buying a new painted tank!
I brought them home, a little more extreme cleaning and one half hand painted with POR15, which went on fine, time will tell if it dries ok (it's cold in the shed!)
The instructions say it needs a topcoat if exposed to sunlight, I will risk a second coat of POR15 only.
Re advise on paints: I realise that a lot of people have different loyalties to their favourite paint manufacturers. After buying the POR15 many years ago, I then came across a comparison test with a Rustbeater paint, where the latter's product came out far better. I think it was Practical Classics, and that award is mentioned on the Rustbeater website, but I cannot now find the actual review. However, for me. the convenience that POR15 is not mixed and it came in handy small tins, so will be much easier for touching in that self inflicted damage.
After reading the review I did buy the Rustbeater product, after using it on the chassis, before getting that powder coated. So I have loads left, free to a good home, if anyone in the Norwich area wants to collect it. Posting it would be too painful..
Note to self: keep these ramblings briefer