Gidday, Don’t have it in front of me but the "Model Railroader" December issue has an article on weathering inside hoppers. There is also this site…
http://model-railroad-hobbyist.com/node/4678
and in answer to a question I asked recently here…
http://cs.trains.com/mrr/f/13/p/211291/2316861.aspx#2316861
there is a link to this…
http://www.shorpy.com/node/2799
…which I think is a great photo showing , amongst other things, the inside of prototype hoppers. I don’t think that the weathering inside the Difco cars would be too much different.
Cheers, the Bear.