You don't say whether the boards are in / out of the truck, or what the finishing issue is. Guess you could use a wire brush or wire wheel to take the crud out. Could use a thin piece of sheetmetal to protect the edge of the groove to keep it square.
Could also use a router / edger to clean up the cut.


1940 Chevy KC 1/2-Ton
In the Stovebolt Gallery