The amount of time the hard coats need after application before it will be ready for sanding will depend on several factors. Allow the coating enough time to cure to a desirable hardness for sanding. The formula will change the cure time, so brushable hard coats, because of their pot life, will need more time before sanding to cure. A high pressure hard coat can typically be sanded shortly after application since it cures in seconds (if you’re working in a temperature-controlled environment).