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Once you have your plaster poured, you simply place your balloon or ball (if a ball is used it would be advisable to seal it with soft soap before use), into the plaster to half cover the ball. You can tape it into place while the plaster heats, cools and sets.
Remove the ball or balloon and container, scrape off any rough spots, leave to dry in a cool dry place for a week and it will be ready to use.