I bought a near new LS-7 for 39k because the word was, “I had a climb problem!” Got it home and in the shop and found a big spar depression on the bottom of both wings! Couldn’t bring myself to jump on this new wing with 40 grit, so I carefully marked out the low areas which were behind the spar up to an inch wide and 7 to 10 feet long! Masked the low areas off and then sanded them with 220 grit to give the gelcoat “tooth”. I carefully leveled the wing both fore/aft and span-wise.. Mixed up a batch of gelcoat and brushed in everything with repeated coats until it was thicker than the depressions. Gave it a minimum cure time, about 4 hours as I remember, then wet sanded the edges with 400 grit while the stuff was green and very soft, switching to 600, then 800 as the depression disappeared. Buffed everything out after completely cured. She climbed with everyone else as far as I could see. Loved that bird, should have never sold it!