Ive got some stone windowsills to repaint,they were done in black gloss this has been removed and they are gona be redone in black masonary paint,the sills are a bit dusty when you touch them do you think they should be stabilised first,also one of the sills I couldn't remove all the paint off and it still has bits of grey u/c on it I've rubbed it down thourghly will the masonary paint take to this ok many thanks