I am wondering why you want to seal the slate? I would not recommend sealing slate in an exterior application for several reasons.
1. Slate has a low porosity and really doesn't need to be sealed
2. Sealing slate can cause all kinds of problems. I have seen sealed slate prematurely start spalling. Even if you use a breathable impregnator, it will still reduce the amount of moisture that can escape from below the slate causing all kinds of problems
I would recommend the proper maintenance program to keep the slate clean rather than trying to seal it.