What causes a mask to crack?

Latex masks are a natural material.
Latex rubber (a type of tree sap) is processed with ammonia and other ingredients which is placed in a mold, and is allowed to dry to create a mask.
Latex masks (and foam) have elastic qualities because of long molecular chains that allow stretching of the whole piece. What happens isn't a loss of moisture, but a chemical reaction that actually alters these molecular chains. The major enemies of latex are oxygen, UV light, smoke,and dust. These enemies cause one of two problems. 1- Either a breakdown of the cross-linked chains, which causes the latex to get gooey, and fail. 2- Or a continuation of the cross-linking process, until the mask becomes hard and brittle.
The only thing that Silicone, or Armor-All will accomplish is that it will make the mask slippery, and that will make the mask un-repairable. Nothing sticks to silicone. No glue, no paint. Nothing. So what you end up with is a mask that is still hard as shoe leather, and if it's cracking, un-repairable.
If your mask is becoming stiff and brittle, I DO NOT recommend using Armor All on it to attempt to soften it. It won't. And you'll be left with a mask you can't glue or paint, and it will be unsuitable to be repaired.

The Solution.

For any mask, a crack is the step before a serious tear. This doesn't mean that it's not still a keeper, it only means it should be placed on a stand and displayed. It should no longer be worn.

To fix a crack, use strips of 1/4 inch masking tape, and tape the hole together. From the outside, using the tape only to keep the sides located.
From the inside of the mask, spread yellow contact cement around the crack and about 1/2 inch on either side. Allow this to dry.
Cut a strip of paper towel, cheesecloth, or muslin as wide and long as the glued area inside the mask. Lay this on clean waxed paper, and paint yellow contact cement on one side of the paper towel, cheesecloth, or muslin strip. Let this dry. Carefully lay the glued paper towel, cheesecloth, or muslin onto the glued crack. Press firmly and remove tape. Brush the paper towel, cheesecloth, or muslin patch with more contact cement. When dry, dust with baby powder.

Now place the mask on a display stand, and enjoy. (Be sure to support it well from the inside, by lightly stuffing with plastic grocery bags if neccessary.)

This may not be completely perfect, but it will help keep your treasured mask together.

As far as a cosmetic exterior repair you CAN use an acrylic surface building medium. However this won't match the texture or color, but it will help the overall strength.