As others have mentioned, I think the easiest and cheapest solution is to just use a chainstay protector that covers the blemished area. We can easily do the cosmetic work, but you're looking at a minimum of $75. Besides, the blemish is in an area that's very hard to see and can easily get damaged again.
If you want to take on the repair yourself, the procedure would be to use 150 grit to remove the chipping/flaking area so that it has a perfectly smooth, beveled transition to the non-damaged area around it. Feel out with your finger. If you can feel a lip from the sanded area to the undamaged area, it will show up in the finish. It's got to be smooth. Once you get it smooth, then use 320 grit to sand out the 150 grit scratches. Then use an automotive grade clearcoat to finish. I would recommend airbrushing, but if you're careful you can probably rattlecan it. Let the repaired area cure. If you can heat it up to 130 degrees, it will cure in about 5 hours. At a temperature of 70 degrees, it will probably take a day. Either way, let it sit overnight. If there are any runs, wet sand them out with 600 grit, then use a polishing wheel and a good quality polish to shine it up.
Like I said...just put a chainstay protector on it and save yourself a bunch of time and money!
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