It is safe to use high temperature paint inside a fireplace, but the surface needs to be clean before the paint's applied or it will not bond properly to the surface.

Thank you for your quick response, I appreciate it. Besides a very thorough vacum job, is there any other 'prep" work I should do before painting?

Thanks again!

Scrub with a wire brush as you're vacuuming. Wash with a strong solution of tsp and rinse well. Allow to dry for 2 days or so. Spray on 2 or 3 thin coats, allowing drying time between. The surface does not have to be immaculate. Removing loose stuff and surface soot is enough.