Have a look here:

http://www.metalscraping.com

This is an excellent site that covers the entire subject of scraping in detail

Mike

Mike – We were taught scraping at apprentice school. We were each given a couple of billets of cast iron 4" dia. x 3/4" thick. The idea was to scrape one face on one billet to so it was true to a surface plate if my memory serves me correctly. Then the second billet had to be scraped to the first. "STUARTS" blue (it got everywhere, and I mean everywhere) was used to reveal the high spots for scraping. A successful conclusion to the exercise was when the two scraped faces were "rung" together and adhered enough so that one would support the weight of the other.

Even at fifteen years old I found the process enjoyable unlike the 'orrid square in a square or the bone crushing chiselling exercise. Turned out to be pretty pointless though as I was never called on to scrape anything for the rest of my career.

Rik