Ok, I talked with my grandad, the master wood-grainer, and he said the secret is really getting the second layer of paint mixed right. First, of course, prepare the piece to be painted, sand and clean it well, then hit it with a layer or two of spray paint. Our choice was Krylon Brown Boots. After letting that dry fully, mix up your "graining" paint. We used burnt umber and plain black oil paints, if I remember correctly. DO NOT mix it thoroughly, as the swirls and streaks of the varying colors make up the wood-grain effect. I would suggest getting some practice in on a piece of scrap wood or metal. After dabbing a medium-small fine brush in the mixture, you just drag it lightly across the base coat, making sure you are going lightly enough to skip spots, and not covering the whole thing. Of course, you should try to drag in the same direction, with the occasional swirl to simulate knots or imperfections. It takes a lot of practice to get it looking nice, which is why you want to practice on some scrap first. Afterwards, we used several coats of polyeurethane to seal it.
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I see now that Mr. Skinner already supplied a fine answer, so hopefully between the two of us, you guys can get your dashes, trim, garnishes etc. looking like they were carved from mahogany in no time!