Craig,
I learned it from an article published back in the 70's by noted modeler Dr. Wayne Wesolowski. "Dr. Weso's Weathering Goop" - Railroad Model Craftsman, September 1977, pg 38.
I got to meet him during the '94 Denver convention Saturday night windup as he had noticed my C&S coal car, asked what I used to weather it with. He was very pleased when I told him.
What ever percentage he said worked for me but like I said it does fade with time. My mid-80's stuff is even lighter. The druggies down here have made the procurement of Isopropyl very difficult; to get mine, I had to take the bottle with the India Ink already in it plus the '77 RMC issue to our local Chemist, he spent more time reading the article than serving some customers.
Built for our Easter National convention here in '94,
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hasn't been left in a sunny place either just "weathered" Weathering. The wood in the carbody is Macrocarpa, known up your way as Monterey Cypress.