Put some 400 grit on your sander and knock down all the high spots, don't get too crazy, you are not going to sand that disaster back to level- too risky.

Now take some well worn 600-800 grit (worn so the back of the paper is soft) and just using the tip of your finger, degloss all the low spots and depressions. make sure nothing is glossy.

Clean with Naphtha

Now, using the same brand clear as before (this is important), go ahead and flood out those craters.

When this all dries, NOW you can sand it flat and buff it out.