The inevitable nicks & scratches can happen to your car.
But sometimess, a person can be walking by... with a key in hand.... scratching from front to back...
I couldn't justify spending $$$ to respray, but on the other hand, it was so difficult to look at.
If this happened to your baby, here's a quick way to "remove" it.

Disclaimer:
This is not an instructables on how to achieve a concours finish.

Tools required
Masking tape
Various grits of sand paper (**600 and higher Automotive only)
Clean bucket of water
Spray bottle
wash sponge
Microfibre cloths
Rubbing/polishing compound.
Wax
Patience

Step 1: Mask

Mask about 1" away from the scratch.

This will be the perimeter you very lightly wet sand within.

Step 2: Start Wet Sanding

With your bucket of clean water.
Wet the sand paper.
Spray the body panel with water.
Start lightly sanding.

Notes & Tips:
This was my first time doing this.
I found that starting with your highest grit (I only had 1000)
allows you to get a feel for how much clear coat you are removing.

If you find that your highest grit is filling up fast.
Try using a coarser grit (ie:600grit) with less pressure

It's not recommended but you can get away without using a sanding block.
After all this is a DIY and in reality you are blending the scratch.

While you're sanding with one hand, spray the surface with the spray bottle.

Use the spray bottle to clean your sand paper.

Step 3: Take a Step Back.

So you're sanding right?...
You're in the thick of it.
You're focused on that square inch of space.
Stop.
Take a step back.
Look at your work.
Did you take too much off?
Is it time to switch to a finer grit?
Are you done?

Step 4: Remove Masking Tape and Inspect Work.

Remove masking tape
Spray area clean with water
Dry with microfibre
inspect.

Notes:
The hazing is ok as it will be removed with the rubbing compound/polish.
The area where it gets a bit darker was where the key went through the clear coat.

Step 5: Polish & Wax

Polish with rubbing compound/polish.
I didn't have a buffer at the time.
So this was the first pass by hand with microfibre cloths.
I'm sure you could achieve better results with a buffer.
Lay on some wax.
and voila...