What I used to do was put some paint thinner in a quart jar, about a third of the jar and then take styrofoam egg cartons or other styrofoam works too (all the same color) and put them in the jar. The paint thinner will melt the styrofoam and the mixture will become thick. Keep the lid on the jar so the thinner doesn't evaporate when your not using it. Once you've achieved a good consistancy, use a paint brush to apply it to your project.
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