My teenage daughter was at college and parked her car under trees most of the year. As a result the car collected a lot of sap. Her roommate told her to remove the sap with fine steel wool. As expected, her 1999 Nissan Altima is now covered with abrasions from the steel wool. What is the best way to try and restore the finish?

LOL sorry its kinda funny.

Hopefully she didnt scrub too hard taking the clearcoat (cc) off.
Id lightly try to take those scratches out with 1500G wet, again just enough to take out the wool scratches. Some may recommend to move onto 2000G next to take out the 1500G scratches but since you may not know how much cc is left Id probably just move onto buffing it with some 3M Finisse It II. That shld bring it back to life.

But dont go just by my word let someone else chime in as well cuz Im not a pro.

how can I tell if the clear coat is gone?

If you use the 1500 grit as suggested just be on the lookout for the color base coat showing up on the sandpaper. If it only looks whiteish or clear as you lightly wet sand you are definitely not through to the clear and should proceed with caution. There is a very good chance that the basecoat is intact.

what color is the car? hopefully NOT silver cuz then u'll have a harder time identifying the whitish sludge from the CC vs the color silver.

also fwiw if there is ever a next time tell her to just try washing the car with a good car soap. A few yrs ago I damned nearly tore my arms off using a tar/bug remover was taking waaayyyy too long. So just for the heck of it I tried washing the car with soap and it washed right off I mean it literally came off MUCH easier than the bug/tar remover was doing.

Also, I've used baby oil, and WD-40. Worked great for tar and sap.

clay bar