Plate filler material

[BSimon999@AOL.COM]

I understand that the black filler on the old cast iron plates, under the 
gold paint, was made from slate dust and boiled linseed oil.  Looking at a 
number of old plates I thought that the mixture might not have been thick 
like window putty,  but rather like a very heavy syrup. They would pour it 
on, let it flow heavily over the casting, then let it dry, possibly sand a 
bit, then paint.

Is that about right? Please correct my assumptions if you know what was 
actually done.

What materials do they use today? ( I know many use better castings and no 
fillers at all.)

Bill Simon
Phoenix