I hit a square steel pole while reversing out in a tight place, turning as I reversed.

The pole hit the mudguard right where the VOLVO badge is (1986 360GLT). There is a vertical line of impact up and down in that spot.

It's not too bad but is beginning to rust after a few weeks, also under the black plastic wheel-arch trim where the paint cracked. I would say the maximum depth the panel has been pushed in is about 20mm, less than an inch.

It doesn't seem very accessible from behind.

Questions:

Must the guard be removed?

If so, is it better to get another one?

Plan B:

sand it back, apply some rust reverser acid stuff (oxalic acid??), paint on some white Kill Rust. Put up with it.

I asume you are referring to the front wing.

If that is the case it does bolt on with a number of bolts along the top 3 behind the door and couple in behind the bumber.

The wings do commonly rust (at least over here anyway) so it would be worth buying a new one and getting well painted and fitted.

If funds do not allow then a knock back to shape a good clean back some good primer and a coat of paint should make it presentable again