Definitely lacquer thinner. If sanding is necessary, I wouldn't go heavier than 320 grit, and up to 800 for finishing. IMHO steel wool would be better, use 0000 grade, the finest. (0 is coarse, 0000 fine.) And if sanding must be done, always sand with the grain of the wood, never across it. Sanding across the grain tears the grain and leaves a rougher finish, as well as working dust down into the grain much more than sanding with the grain. Use tack cloth after sanding to remove as much dust as possible.
Try this first. Spray on -lightly- a fine mist of lacquer thinner. (You can get pump sprayer bottles that will work great at department stores like the dreaded wally world. Don't use an empty 409 bottle or such, the leftover chemicals may cause problems.) It should "melt" the lacquer, which will then slowly reharden. That will take a couple of weeks. I have a friend who does nitrocellulose lacquer guitar finishes, he insists on 2 months cure time before he will give the guitar back. It will not be an overnight project, don't plan on even touching the guitar for at least 2 weeks. If it does not harden after a couple of weeks, it was probably a bad batch of lacquer. In that case go back to your original plan, strip and refinish.
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