Best sealer for exterior terracota tiles?

I have a fully exposed balcony with terracota tiles. I've just finished cleaning off 5 years worth of dirt and mould, so now I'm thinking maybe I should seal the tiles in the hope of decreasing this problem in the future.(?) What is the most durable sealer in this case? The tiles receive full U.V. from the sun, so I'm wondering how long sealers can realistically last. Miteq penetrating sealer was mentioned elsewhere, and I've just had a look at their product overview pdf file. Are they the best, or should I consider others? I don't care about cost or effort. I want an excellent long-lasting final result that won't change the current tile appearance excessively.