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I have done this type of repair multiple times below and above the waterline with marine tex. What works really well is to take some good blue 2 inch masking tape and mask the area with 3 layers of tape exposing only the, "chip". Mix and spread the marine tex and let dry overnight. Now get a block with some sand paper starting with 80 or 90 grit and sand the area until you start to sand thru the first layer of blue tape. Then switch to 120 grit and sand thru another layer of tape. Finally, use 180, 220, and then wet sanding 330 grit to almost thru the last layer of tape. What you end up with is a nice localized repair without sanding scratches or marine tex on the surrounding gel coat.
I am really fastidious so I go one step further wet sanding up to 800 grit and then buffing the area with a Makita high speed buffer and some good 3m compound. If you go this far you will not be able to see the repair from 2 foot or further away. You might notice just a slight color variation in certain lights but no real evidence of the damage.