Cleaning a Negative Cast
Prior to sealing the negative, you must clean out any plaster residue from the inner surface. This will ensure a smooth finish on the positive cast. Dumping out any loose debris and scrubbing the inside of the negative with water and brush works well.
Sealing the Negative Cast
The negative molds are split along the anterior surface and around the dorsum and plantar aspect of the foot. This seam allows the negative to be split open and removed from the positive cast without breaking the foot off at the ankle. Before filling, this seam needs to be sealed. This is done using duct tape
When using duct tape to seal the cast, you must make sure that the seam is aligned correctly and pressed together as tightly as possible.
First seal along the edge of the seam longitudinally.
Use smaller pieces of tape along curves like the dorsum of the ankle. This prevents the tape from bridging and allowing leaks.
You can also cut into the edge of the tape strips to help them lie flat as well.
After the seam is sealed a few circumferential pieces of tape will stop the negative from splitting open once the wet plaster has been poured into it
After sealing the cast, pour water into the cast to check for any leakage.
If it leaks, apply more duct tape to block off the holes.
Once there are no more leaks you know the seams are sealed.
Soaping the Negative*
Soap the inside of the cast by adding soap and water, or applying a thin layer of soap with a paper towel. This ensures an easy removal of the positive cast.
Just one or two pumps of soap is sufficient.
*It is a good practice to apply soap or other separating agent prior to filling casts in the real world, this will save a lot of time when stripping the positive cast.