Clay question

[Jersey John]

Probably stupid question, but how can I tell if I should use medium or fine grade clay. I'm a home guy so I've only had one type of clay, but I bought a nanoskin clay sponge recently to speed it up and it came with both fine and medium. Also is it harmful to use a medium if it doesn't need it.
Thanks in advance.

[Setec Astronomy]

You'll know you need the medium grade when the fine won't remove the contamination or it's taking too long. Yes, using the medium when it doesn't need it risks unecessary marring.