Re: iron filler
I believe the filler used in olden times was made from a mix of fine sand, limestone, maybe some clay for spreadability and water glass solution (sodium silicate) The water glass, as it dries and/or reacts with the filler, bonds the grain together and also sticks to the iron. I restored several engines with this schtuff on it and on occasion, found bits of iron, copper filings and other floor sweepings mixed in. Once you paint over it, it is there for the duration. One 100 year old flywheel I worked on took me several hours and 4 grinder wheels to get it off. About 75% of the original material was there and this old girl sat out for who knows how long. The water glass has an affinity for cast iron and forms a chemical and physical bond.