Andrew, I plead ignorance -- I've only ever scraped steel and aluminum. All the ways and slides on the Gingery are steel.
In my imagination cast iron scrapes as nicely as it machines -- happily if grey, and past the skin. But I dunno for real.
Cast aluminum needs a low angle and can take longer than you expect, even though soft. It can chatter and dig.
Dawesy, I don't know how they scrape long beds and prismatic ways (I think that's what they're called) but there's probably a video or web page devoted to it somewhere now. I have the opposite problem -- a 4 foot straight edge in a 6 foot by 8 foot shop, and a 2 foot lathe.
Actually, I've switched to a 2 foot cast iron surface plate which is better all around, for this bed -- it does 2 rails at a time, and shows twist without checking with machinist level, etc. I just hope the plate is true -- I bought it at auction and it looks venerable in age. Ive tried switching the bed around to different orientations, and the marking seems consistent. But that isn't an absolute guarantee. I thought of testing the straightedge and plate against eachother, but things are too cramped to be able to do that now.
A 6 foot straightedge must be quite a massive object..... how did they do 10 foot lathes? 20 ft lathes? There must be a way, with shorter reference surfaces.
ps.....didn't someone here cut down a straight edge awhile back? And scrape a whole kit of reference tools......that cut would take a lot of courage.....