… could you use it on a steel bike frame? Does anybody know anything about it, price, toughness etc?
I’m sure I read somewhere (maybe the Guitar Ted website) about stuff called gun-kote which was used on a frame.

Its called Bluing, thats the tradiotnal method anyway, it’s not tough at all, in fact it would be pretty crap on a bike:

http://en.wikipedia.org/wiki/Bluing_(steel)

I suspect that there are more modern methods that are robust though.

Went back and found the article on Gun-Kote on the twentynine inch site. Seems to be very good, very tough and very light.http://twentynineinches.com/2010/09/07/gun-kote-frame-coating-final-review/

Of course it would probably be very expensive even if you could get someone to do it in the UK.

or it’s called blacking. it would change colour or wouldn’t take round the welding/brazing. to do it properly you would need a big tank for the caustic (rather than the wipe on paste stuff)and for the oil bath afterward. it’s a stinky dirty job plus the metal needs a very high polish beforehand and a thorough degrease. i would worry about all the little holes in the frame and getting the solution out afterward.
it would scratch and look scruffy unless lacquered and then you might as well use coloured lacquer over metal for a similar effect.

Doesn’t stop rusting – you still need to wipe down with an oily cloth after every use.

Acid / fat from fingerprints will mark the surface if not cleaned, as well.

i clean my gun after every outing inside barrels and outside and still you get imperfections easily and need to reblue c. every few years depending on use, IMO i would not bother, go for black / midnight blue anodizing!