I have a Planet Bike Cascadia Fender that needs to be trimmed to fit at the chainstay bridge. Probably only need to take off a couple of millimeters. If anyone has done this, what is the best tool for the job?
I have a Dremel tool, but it has lots of attachments, not sure which to use...

You could use the cylindrical or conical metal burr bits, or if you have it, the side cutting bit that looks sort of like a drill.

I also have a dremel, but for job like this I'd just pull out a hand file. A hand file offer much more control, with less chance of cutting small gouges as you move along. Use a half round file, and if you can find it, a half mill (teeth are cut in one direction only) will leave smoother edges.

gouge section to be removed with a box cutter, then bend offending bit in direction which opens gouge with pliers. It should come clean off with no sharp bits.

high speed cutting tools tend to melt polycarbonate.

if only a few mm, then coarse metal file.

Have trimmed lotsa fenders, most recently several Planet Bike models. Try mounting fenders on recumbents without being creative.
Side-cutting diagonal pliers are good for making cuts at the edges. Snips or shears can work to make controlled cuts across the fender.
Just a couple mm at the edges? Sandpaper? File? Rotary sanding wheel on your Dremel? Or whittle the edges down with a sharp knife blade?

I keep a nibbling tool around for things like this. Works on plastic and metal, you can make very straight and even cuts, and they're cheap.

Soften it up a little with a heat gun and squeeze / shape it until it fits.

I used sheet metal snips. Easy to use, can trim a mm at each cut, and does not require power.

I would use the drum sander - course grit. Do not use to much pressure as the sander will load up with plastic. And if you want to get rid of that ugly logo, take a little solvent on a rag and rub the logo. I wish that PB made a cool looking logo to put on thier stuff.

Tom

If you have access to a bench mounted disk or belt sander a medium grit sandpaper will take off a couple of mm quickly and fairly cleanly. Clean up the edge by hand with some 400gt and 800gt finishing sandpaper and then polish the edge with some plastic polish on the buffing head of the Dremel tool. Should end up looking like a factory edge.

I used the Dremel with a small sander drum on my sks chromoplastics and one of the metal nibbling bits. For finer work, some sandpaper wrapped around a dowel is better.
You need to shape the two cutouts accurately so it still has some grip outwards to the chsinstays and can hold itself in place. I used a small ziptie through a hole 5mm from the tip to keep the fender in place.

Sharp knife?

Dremel with side cutter works great, but be careful, it's also fast.

Thanks for the replies. I had to Google a few of the tools mentioned...now I know what a nibbler is!
When I saw the idea about the heat gun, I decided to try installing it without any trimming, and it worked!