I was at Gunter Pfrommer's a couple yrs ago and he showed me how to do the "file cut" matting used on the ICD's and some European guns. The tool was literally made from a single cut mill file. The piece of file was brazed to a shank and ground so as to form a chisile shape with the sawtooth pattern on the end. This would leave those lines as the tool was chased across the rib. He demonstrated it quickly for me. Simple stuff.

Doug,
I got involved in helping a friend build a RV-3, later a RV-4 and some extra -4 wings, many moons ago. I did the first flight for a friend on his RV-6 a long time ago. Flew a bunch of diff homebuilts over the years. My Pitts is collecting dust in a hangar. Way past time to sell it.