Answer: With just a little effort you can reduce the impact of scratches and with further effort you can usually get rid of them completely
Ok... so your Hobie kayak fell off the back of a truck and now it's got a deep unsightly scratch or gouge. Fortunately it is possible to minimize the deformity with a little effort. With even more effort its actually possible to hide the scratch almost completely.
Using a heat-gun (or a good quality hair-drier might do) hold steady heat onto the scratches just long enough for the sharp lips of the gouges to melt back upon themselves and back into the gouged groove. Use a smooth surfaced object (such as the underside of a teaspoon) to help push the melting edges back in lightly, working your way from one end of the scratch to the other. Be careful to avoid applying too much pressure and be especially cautious about applying too much heat - you'll know you're getting close to doing just that if you note the plastic surface change glossiness or shape in any way. This step alone is usually enough to obscure a mild scratch (demonstrated in the photo below) but further steps can be taken to clean it up even more.
The next step is to take to it with a block & sand-paper, starting out with 600 grit, sanding it back lightly. Don't work it too hard and be wary of taking away too much plastic in any one area. Better end results will come with more time taken, stepping up in fine grades of paper for best effect, 800, then 1000. To finish it off you'll want to use some buffing compound, applied vigorously (but not too vigorously) by hand with a clean rag, or with a buffing machine. This will put a nice polish around the area and should help to smooth out the gouges even more. Be wary of going too heavy-handed with the buffing though - it's possible to take off too much.
Much the same technique can be used to bring an entire hull back to better-than-new life, although obviously it's a longer process. Usually, however, in these cases its more a matter of many small scratches to deal with, as opposed to a few really deep ones. Perhaps it'll be a bit of both. In any case, the heat treatment should only be reserved for the deepest gouges. Sandpaper & buffing will take care of the rest nicely.