I am about to fit a new woodburner and have repaired and cleaned the soot off and sealed with PVA . The rear of the fireplace is still blackish so can i paint this with normal masonry paint, there will be a 6" gap from the woodburner.
You may need 2-3 coats of PVA to completely seal the soot, otherwise it will come through.
If you have done this it should be fine with masonary paint.