Let's say this is for benchpressing. How would I ask someone at the gym if they wouldn't mind spotting me for a set?
Also, is this common and/or appropriate?
Asking for someone to do spotting for you isn't inappropriate(of course), but ask your trainer/coach(if there is one) or someone you have become friendly with, rather than asking some random person. Korean people are not that used to talking with strangers, and will probably show bewilderment. I would recommend that you either take a friend with you, or find a gym with a trainer.
For a way of asking, I am not sure if there is a specific word for spotting in Korean. Ask for some help, instead.
벤치프레스하게 좀 도와주세요.
This might seem too simple and unclear, but a trainer will understand.