First, it doesn't "dry", it is a chemical reaction which is why you need the correct ratio of filler & hardener and to mix them intimately (why they are colored).
The biggest tip I wish I'd known years ago is to get a "cheese grater" type wood plane (Ace or Home Depot) and level it off as soon as it starts to harden. It will shave off easy. If you wait until it is hard, you will spend the rest of your life sanding. Don't mix too much. The can says "15 min" but I get 5 min even on a cold day, just enough time to mix and make 3 or 4 passes. Don't worry about the first coats being smooth. As suggested, use very coarse sand paper or it will gum up. After the final coats and fully cured, you can sand finer and it should come off like powder. If still gummy, you didn't use enough hardener.
If covering large areas, use a wide putty knife (12") or even a steel strip. Some people spray a thin coat of gloss black paint to check the reflection, then later sand that off. If not, it may look great until painted and you see wavy parking lot lines in the clear coat.
