Greetings ... a few years ago I saw a video where the guy was using Elmer's Washable School glue for a chipping medium, he also used enamel paints for the top coat.  I finally got around to trying it this weekend and I really like the results.  I used it on some scratch made1:35 corrugated metal roofing which I'll also real quick try to explain too.

For the corrugated metal I use the adhesive backed peel and stick foil HVAC tape, NOT DUCT TAPE!  I peel the paper backing from it then stick that to kitchen aluminum foil.  That covers the sticky and gives the metal the right feel and weight, I think.  I then use the .080 corrugated siding from Evergreen as a template for the grooves.  I use an old paper bone to get form the grooves.

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I then prime that with RustOleum red primer.

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To get the rust colors I use a variety of cheap craft paints that are sponged on and wiped on.

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After this dried I used a sponge to dab on the washable Elmer's, I did NOT use a clear coat over the craft paint, I've found these paints to be pretty sturdy once they dry, but I'm sure a clear coat wouldn't hurt.  I then used a variety of Vallejo blues and greens airbrushed over the glue after it had dried completely.  I let the Vallejo dry as I cleaned the airbrush and made some tea, then I grabbed a wet toothbrush and started gently scrubbing the paint.

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I'm super happy with the results, I think I'll be sticking with the Elmer's going forward.

Erik