I accidentally sanded my print a little too much, I don’t want to reprint it since it took 12 hours, is there something I can use like clay or putty that I can apply to readd the rounded art that is sandable and that I can put primer on and paint over?
As others have said, there are tons of fillers (Bondo, Rage Gold, glazing putties, etc) that you can use for this. If your part is flexible make sure you use something that can flex, otherwise it will crack.
That said, consider the cost of doing this in terms of both time and material $$ first and then weigh that against simply printing the part again. If you’ve never worked with fillers before you’ll have to do the apply, wait for it to dry, sand, apply more loop a few times. It’s not necessary a bad skill set to learn, but it will take some time/effort and the materials aren’t free.