Yeah, Hyo is right if it were a silicone mold. However, since you are wanting to make latex masks, you are on the right track. Ultra Cal 30 is used for creating the hard mold used for slip casting a latex mask. So yeah, You will need a sealant. I have Krylon Crystal clear and You should be able to pick it up at either home depot or a discount auto parts place. I got mine at AutoZone if I remember correctly. On a side note, I have heard of people using silicone molds to make latex masks, but I've heard its a real pain. THe latex tends to not stick to the silicone.
Edit: You will also need a release agent on it too.