I scraped the watch lathe bed today. I was lucky enough to find a Biax scraper on eBay a while ago so it went pretty fast, less than an hour. The process of scraping is pretty simple to say but takes a lot of skill to do. You just put some Prussian blue or high spot paint on a surface plate, put the surface of the object you want to scrape face down on the surface plate, move it around a little, then just remove the blue by scraping. The blue will be the high spots. I will not go into scraping very deeply, that is a subject for a book, namely "Machine tool reconditioning" by Connelly. There are a few quick tips. Make sure you don't have any big burs on the surface you want to scrape or you could gouge your surface plate. After each session of scraping, run a file lightly across the surface to remove any burs that are brought up by scraping. Make sure you put even pressure on the surface that you are marking. When done properly, the surface you scrape will be just as accurate as the surface plate you used to mark. With the three plate method you can easily get an accuracy of 50 millionth of an inch. This was done in the 1800's.
I think I did about 10 marking and scraping steps even though there are only 6 pictures. You can see the dark blue on the ends of the lathe bed at the start. The blue get lighter and lighter after each step, not because there is less blue left on the surface plate (I actually added more about half way through), it is lighter because the surface is getting flatter and flatter and there is less room for the marking blue. At the last picture, the lathe bed is flat within the limits of my surface plate +-.00005". I have a black granite surface plate. The marking blue will permanently stain your surface plate. You can barely tell on my black plate, but I would think twice about using a pink surface plate.