NavList:

I know the post is quite old, but just in case you were still curious, the reason the radiation reached the detectors in the airport but will not trigger new fluorescent paint is that the (as you rightly stated), Radium emits, Alpha, Beta and Gamma. The Gamma will be what the detectors would have picked up - the alpha although extremely plentiful has an aburdly short range and is stopped by something as simple as a thin sheet of paper.

So although the radium is still almost as active as it was when it was first applied - and emitting enough Gamma to upset the Airport folks, because the alpha radiation is unlikely to leave the paint, it cannot activate anything new.

Arguably and at some risk, you could try to remove the radium paint and using solvents, attempt to strip away as much of the old paint / Zinc sulphide and THEN blend it with new lume.

I have been collecting radium from old watch hands to make small geiger counter "Check sources", but there's not much around these days. Harmless enough so long as you don't eat it or inhale it! :)

Guy