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Later coats of latex need to be thicker to fill in undercuts. The inside of the mold follows the detail of the model, but the outside of the latex should be smooth and rounded.
To thicken the latex for this application, CECO powder has been developed. It mixes with the rubber and it does not have any negative side effects that harm the latex.
Mix the CECO into the latex. Make it into a thin paste, or putty. Mix it thoroughly. When the mixture can be dragged up the side of the mixing bowl without sagging or dripping, it is the perfect thickness.
This paste is going to be used to fill in undercuts. Also, the paste is used to thicken the lip along the base. (This thickness at the base helps prevent the mold from losing its shape.)
Using a procedure of mixing and applying the paste, over a period of several sessions, build up the surface of the mold gradually. Do not apply it too thick, or it will not dry easily. Build up undercuts, and the base and the seam line, gradually. Progressively round out the surface of the mold. Use the paste to fill in undercuts that would become “hooks” if ignored. Any hooks or undercuts will become problems when removing the mother mold. Also, this builds a surface against which the mother mold can be built without gaps. The mother mold will be supporting the latex at every point, avoiding future problems.