What type of tile sealer is used for outdoor projects and will this help to prevent the grout from cracking?
Yes you definitely should seal your outdoor mosaic. There are two types of sealers, penetrating and membrane sealers. The Penetrating sealer actually goes in the grout and fills all spaces whereas the membrane sealer forms a membrane and gives a glossy finish to your grout. Sealants are easy to apply, and both types can be used outside. However for outside mosaics I always apply 2 coats, and reapply a fresh coat every year. By doing this you will protect your mosaics from discolouring and cracking.
