ive got a pretty nasty dent in my rear fender behind my drivers door. I'm under the impression that it's fiberglass and curious if I can just pop the dent out and then clean up the paint?

That's not a dent. That's a collision. The rear fender is steel, not fiberglass. You need some major bodywork here! Sorry to be the bearer of bad news.

Knocking on the door vs the fender it sure doesn't sound the same. Door is definitely metal but the fender sounds more like plastic or fiberglass. I'm not pretending to know for sure but there is definitely two different materials at work here.

You can always do what we did in the old days to test for bondo. Pass a small magnet over it. If it sticks it is metal.

Well, the good news is the fender definitely isn't metal. I was able to get the dent most of the way out. I need to remove the trim piece, which I believe is metal, because it is preventing it from coming out all the way. With the exception of a couple spots that made it through the paint this might turn out pretty well.

I thought that part of the car was aluminum.

Nope. The way I believe it is set up is the doors, hood and trunk lid are all metal of some type and the rest of the body panels are made up of some type of plastic or fiberglass material. The idea was likely to save weight where they could while keeping it safe for impact.

This is what a prior thread says on the subject:

Was in an almost identical accident this year, insurance took care of it. about $1900 in body work and paint and less than a week in the shop