I've just twigged what you were trying to remove...it's the protective layer that stops it from rusting too badly if you're going to leave it anywhere for a while :( my frame has been sat in a damp garage for nearly 2 years and only has a small bit of surface rust in a few places, and nothing a quick tickle with the grinder and flap disc won't sort before painting ;)

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As Jonathan says powder coating is good but only on the finished frame as you can't touch it up like you can sprayed paint.

Halfords primer would be ok, though for the amount of steel you have it will cost an arm and a leg!!!

If you get stuck I have a friendly local (to me) painter who could probably do it quite cheaply. Or there is one place near Mitcheldean I think we have started to use at work for painting.