Mark
riccardo is correct. The police often use a method by where acid is applied to an area where a VIN is removed, ground off, whatever. As the acid reacts differently to the two metals the numbers will start to show through. It works well in instances where the VIN is stamped as the stamping process actually deforms the metal beyond the actual penetration of the stamp itself.
Being welded over might be a bit trickier. The key would be to carefully take the weld down to the same height as the virgin substrate metal. Perhaps then the acid may allow the numbers to come through.
It's a process though, not a procedure. Agencies have experts who us different acids, magnafluxing and other techniques to get the VIN to present itself. I saw it done on a Triumph that had the "pad" on the engine where the VIN goes. Earlier bikes didn't have the pad, so thieves would grind the pad off to make it look like an older t120 Triumph and stamp on some new numbers. The acid test would still expose the old numbers. Neat stuff! Good luck.
And....Where the heck do you find all this stuff!
