[JohnnyK81]

Well I accidentally covered up the gap where the rear quarter meets the rocker panel (67 Mustang), But I'd like it to be a little more straight, anyways.

What's the best way to cut that out, and 'round' it a little bit.. I was going to try a tiny pneumatic recip saw (HF) but decided that it would gouge everything, and not be straight anyways.. Same with a cut off.. Soo.. Any ideas? What do you guys do?

[HalfOunce]

One could rough cut it with a hacksaw blade by hand, you see metal obviously stop. then finish with varying grits of sandpaper.

[JohnnyK81]

I had thought about that.. Is that what everyone does?

[milo]

Use masking tape to define the line on both sides of it.
Then fold a piece of 80 grit sand paper and use the folded edge to cut...

[BEN24]

I have used a square and a scratch awl and then a lite sanding on the edges.

[Runnin'OnEmpty]

A Dremel tool does a good job if you have a steady hand.

As has been said, use a straight edge and tape to
outline the cut; finish off with sandpaper.

[BuickNess]

Ive done this before. What you want is a tiny disc on a dremel just like what he said. have a couple beers to calm your hand before doing it. Atleast thats what i do. Cutting body filler is not a cool thing lol