The question is:
How to restore fins that have some scratches and dents?

The topic probably was discussed already on many occasions...

so I will be grateful for suggestions, references or just provide some tips again.
I have few nice and fast fiberglass fins that developed some bruises,.
I am thinking about painting them with resin and polishing again ( ? )
can somebody recommended filler to fill bigger gaps?
what sort of mechanical tools we could use for the best outcome ? Belt sender , what size grit?
How important is the tip of the fin that did ride few times on the sand and get thicker, blunt ?
Or maybe resizing bigger fins into 1 - 2 cm smaller is option?

If it is tip or leading edge dings, repair them with Epoxy resin or epoxy filler, but just do the very local area where there is damage and fair it into the existing shape. Try very hard not to change the leading edge radius elsewhere and match your repair in to it.

For more general scratching, use a very soft filler paint (3D panel filler from a spray can which sets very quickly) and then carefully sand with 800 or 1000 wet and dry (WET) on a small a flat sanding block untill most is gone and you are almost back to the original surface. It may look patchy and ugly, but it should be smooth. If you want it prettier, spray again a second time and this time don't sand right through. I like the black filler spray coat.