I used the Coat-It on the outside of my hull. Its strong stuff and my boat doesn't leak a drop. Now, Its a bit of a pain to apply it, you only have about 30-40 minutes of working time with it. It is a bit less expensive then steelflex, but also--like Gluvits--requires additional painting to be 100% UV protected.
Its strong stuff, and will work. One word of advise though, if you do use it--flip it upside down a few hours before you open it. If you do not do this, all the graphite will be settled to the bottom and its wicked hard to stir that back into the mixture. A drill stirrer will also be a great thing to use rather then stirring by hand. Also, do not use a normal paint roller--you need to use one that is non-absorbent.