Rust inside frame tubes
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Thanks to George Dockray for posting this information on the Yahoo! Loopframe_Guzzi news group. In George's own words:
I had lots of water and rust in the frame cross-tube where the battery plate rear bolts thread into. For that, after the paint was stripped and the suspect areas were visually inspected, I tapped with a center punch all along the tube looking for thin spots. After satisfying myself that the rust had not eaten too much of the tube, I dried the inside of the tube with compressed air then heat, then poured inrust-mortrust converter to the top of the threads, then let drain and dry. Then filled with linseed oil and drained.
This method is basically what you do with suspect steel tube fuselage airplanes. With airplanes though, the tubes are much thinner wall. With a loop-lump, you have a lot of material to get rusted out before I'd imagine it to be a problem. Still, the center punch test will tell you the story.
If you're powder coating, you can use JB Weld to fill in the punch marks and any rust pitting. It's good up to ~600°F so will be unaffected by the heat during powder coating. Sands nice too.
Another thought from Mark Etheridge of Moto Guzzi Classics, also posted on the Yahoo! Loopframe_Guzzi news group. In Mark's own words:
Drill a couple small holes on the bottom of frame rails and forgetaboutit.