antiquing soloution

Crackling medium sold at craft stores.
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But , to dull the colors after the art work is done, apply a wash of burnt umber (brown) thinned to the consistency of ink. Brush on, wipe off, the color goes into the cracks and dulls the bright new art.

Paint your sign, beat it up, with chains, drag it behind the caar on a sandy road (this doesn’t take long or far to go, but is very effective), or sand off some of the paint by hand, then wash it with brown or black.

Practice, of course, on something as close to your finished product as you can — a piece of scrap wood with some leftover paint on it works well. Once your scrap piece is dry, put some black paint on a cloth and rub it on. Try it a few times to see how quickly you need to rub it off, etc. Once you get an effect you like, do that on your real piece.