Sealing coarse concrete rooftop?
All sealers that I have worked with require a clean, dry, scale free, dust free surface. A vacuum cleaner should do the trick.

As far as pre-coats, and "how to apply", you need to follow the directions on the particular sealer you're using. They all have their own specifications.

Many sealers can be painted over, check the specs for your sealer.

Certain local codes do not allow renters to preform any work on their units. Roofing work is dangerous so be careful.