The roof (sans roofwalk) has been completed.
I started by filling in the crest of the roof with wood filler, sanding smooth, and then coating in the usual sanding sealer and 00 steel wool. I cut a strip of paper the length of the roof and laid out the location of the roof panel seams on it, and used it as a guide to mark up the roof itself with a square edge. 
The seams themselves were applied in two layers - a 1/8" wide .020" thick strip as the base, with a 1/16" wide .020" strip centered above it. The strips were cut to overhang the sides, and then sanded flush. The photo below shows one seam completed.
Each seam was then beveled from the roof edge in 1/8". The seams were then coated with sanding sealer.
I always find the roofwalks on boxcars to be easily marred during construction, so I do them last. I'll work on the underframe next instead.
~Chris
