A couple of years ago I bought a set of Indian made WM20 forks, before I knew any better. I rubbed them down and noticed some whitish powder at the welds. I removed this and sprayed them with primer. On inspecting them now I see the whitish powder seems to have bubbled up, a bit like the efflorescence on a battery terminal. It wipes off quite easily.
It sounds like residual flux from brazing which has not been removed before painting. Clean off all the paint in the area and give it a good clean with a wire brush and wash it down well before applying fresh primer.
