Short answer - yes, you can fix it. When I was in A&P (airframe and powerplant) school, one of the exercises was removing a scratch made by an instructor's key in a clear plexiglass window. Some of the techniques we were taught I still use and may be useful to you.
Work from heavier grits to lighter ones. when you switch grits, change directions so you are going perpendicular to the last grit. So if your deep scaring is along the length of the hull, choose your heaviest grit and sand up and down, perpendicular to the scratches. When the previous grit's pattern of scratches is no longer visible, you can switch to the next highest grit. This is why you change directions - if you always go the same direction, you'll never be able to tell which are the heavier scratches and which are the lighter scratches on top of the heavier scratches.
Work to progressively finer grits, changing directions each time. Recognize that each time you switch, you'll need to sand a larger area to feather and fair in the area you're sanding. I like to cheat a little bit and after the coarse grit I'll rub in some thin CA glue to fill a lot of the scratches in and make the medium grit go perhaps just a wee bit faster. I use this a lot when shaping hulls that have a miss-match, and since I want to cut that down I start with something like 300 grit sanding sticks. If I'm not shaping, I'll usually start with some 400 and then move to 600 and then some 2400 or 3200 grit Micromesh I have - it's a really fine grit that's good and a bit of an overkill for the last step, but should give you a good enough "shine" to see what areas you might have missed. Micromesh start sets are fairly inexpensive, or you can buy individual sheets for cheaper still.
I covered this and posted some photos a few years ago in this post, in case the photos are of any use.