Shape the foam blocks into bricks with tapering sides, using a sharp knife. Each brick should be shaped like a long, low truncated pyramid or loaf. Size is not critical as gold bars come in a wide variety of sizes from 1 gm to many pounds, but keep the proportions roughly two or three times as long and high or wide.
Mix a quantity of plaster sufficient to coat one or two bricks at a time in a large bowl according to package directions.
Push a skewer into the bottom of each brick to use as a handle for dipping it in plaster.
Dip each foam brick into the plaster in the bowl to coat it on all sides. Plaster should be the consistency of heavy cream at this point.
Set the coated bricks aside on a wire rack until the plaster hardens. For a complete cure, allow at least 24 hours or until the plaster is no longer cool to the touch when held to your cheek.
Use sandpaper to smooth and even out rough areas once the plaster is completely hard.
Wipe off any plaster dust with a damp rag and allow the wet areas to dry all the way through once more.
Spray the bricks with gold paint untilthey're lightly coated. Repeat several times, using light coats each time and letting them dry a few minutes between coats to produce the best finish.
Let dry for the period recommended on the paint can before using.