You should not sand into the Gel-coat too far. The glass underneath is very thin ~9oz and is easy to damage.
The best thing to do is simply sand the boat with a file board also known as a faring board. This will remove most of the waves that are so common on old production boats.
BTW I used 3 quarts 2 part Interthane by Interlux for my SC 20
Each 2-part kit totals one quart in volume or maybe one litre.
