I like to make signs, so when I saw an excellent instructable on making foam "sandblasted" signs, my ears perked up! I make them too, but have what I think is a simpler method. Check it out.
Step 1: Cut Out Your Letters
In this step, you make the letters to be used for the resist. The letters prevent the foam from dissolving, much the same as in sandblasting. I use my word processing program to print out the message I want to use. The letters are easily cut out with scissors as seen in the picture.
Step 2: Glue Letters to Foam
In this step, the letters are placed where you want them and glued in place. I use regular white glue in generous amounts to insure complete adherence. Don't overdo it though, or you will be waiting a long time for the glue to dry.
Step 3: Ready to Spray
Pick a spray paint that contains acetone, the key to dissolving the foam. This will be listed on the ingredients label as "Ketones". Test the paint to make sure it works.
Step 4: Spray Your Sign
Start spraying as you would any project...not too thick, not too thin. The correct amount is determined by experimentation, but it's hard to over spray. If the paint pools, it will eat all the way through.
Step 5: Complete Your Sign
After spraying as many times as you think necessary, sign is ready to finish. Choices include leaving the resist paper on and painting perhaps, or removing the paper for a more "chiseled" look. Take your pick. See samples for various results. The possibilities for making signs is endless: think tombstones, house numbers/plaques, garden markers,direction signs, and so on.
Second Prize in the