You have to be sure the bolt, water jacket and the cylinder holes do not accumulate crap from the scraping. The bolt holes are easy as compressed air will clean them out. Judicious use of tape will cover the others. When you are ready to bolt down the heads, use brakleen or similar product to clean the bolt holes. Bolt holes need to be absolutely clean. It is also a very good idea to use new bolts and to chase the bolt holes with a bottom tap before the final air blow out. The final scrape of the head and the block really works well if you use a simple single edge razor blase scraper with light pressure.