The nice thing about alginate is that it doesn't stick to anything; the bad part is that it doesn't even stick to itself once it has set. If you're doing a large mold and can't quite mix and use enough to do the whole mold at once, just mix a small amount of this material with water, spray it onto the mold surface where you want to continue, let it set for a minute or so, wipe it off and add more alginate. The two sections will now bond as one.
