"Welded" on? I don't think I have ever experienced this myself. Every bit of dust I've found on the sensors of the D200 and the D3 has been able to be removed with judicious use of the rocket blower. I've never even had to use swabs, fortunately.
I finally resorted to getting a swab kit from Samy's the other day. I had more "welds" than a Chevy Nova going on in there, and refused to have my camera out of my hands for 10 days. Worked out pretty well.