I needed to paint and weather my bridge for my single angle photo shoot and I wanted to try and assimulate some of the bridges around my area. The particular bridge I am modeling is painted silver, but I wanted to paint it black to contrast the rust and weathering (I also didn't have any silver paint). I wanted the peeling paint effect, so I stippled water color masking fluid (rubber cement or liquid latex would work just as well, I just happened to have it on hand) randomly over the plate girder then concentrating on the vertical angles. After the fluid dried I airbrushed grimy black over the whole bridge. Once that dried for about an hour I rubbed the areas that were masked with a gum eraser to get the peel effect. But, it didn't look right, so... After some careful study of my pics I realized that there were a multitude of shades of rust inter-mixed with what WAS black paint (now it's more like chalky grey) into what I can best describe as mottled blotches. Without doing anything to what I had already done, I re-did everything again except I stippled the fluid in different areas while purposely overlapping some of the spots that were masked. I dusted the whole thing down with D&H grey then with grimy black. I waited for everything to dry, then removed the masking fluid. Oh yeah! Now it was looking better, so I repeated the process again, but varying teh masking fluid application. Once I was totally satisfied I reaplied the fluid, but this time I painted a thick coat(s) on the whole girder with grimy black and then dusted it with the grey. The last coat was thicker to get a good peel. It's a multi stepped process, but the effect was worth the effort.