No, you don't necessarily need to replace your CSU assuming there isn't immediate and noticeable damage to the forks.
This is an old question, but there's a good few views on this, and it might be more helpful to give a more detailed answer. I wouldn't recommend trying to sand the burrs initially (or using a blade), as I've found you can cause the coating surrounding the scratch to flake off.
Here's the process I use.
You will need:
- A filler (see note below). I use nail varnish top coat.
- A spoon. Don't use the good silverware.
- Fine grit wet & dry paper. 1500 grit is good.
- A sanding block.
- A cleaning solvent that leaves no residue (isopropyl alcohol, or brake or contact cleaner are fine).
- Clean the stanchion with your solvent. You don't want any degreaser to get on your seals. Make sure it's all good and clean.
- Remove burrs by pushing on the stanchion using the back of a spoon. Ideally, you'll try to push the raised surface back in towards the scratch. As you finish this off, run the spoon up and down the stanchion - you want to avoid any scratches running horizontally. Vertical marks are less critical.
- Clean the stanchion again as before. You want the scratched area to be very clean.
- Add your filler. I'll assume you're using nail varnish. Don't add a
huge amount, but keep in mind that most fillers will shrink slightly
as they cure, so you do want to fill a little bit above the
- Allow your filler to cure. Don't rush this bit. Don't try to use a heat gun or do anything clever. Just give it time.
- Wet polish the excess filler off using wet & dry paper. Only polish up and down. Wipe away the sanded material as you work. Doing this in a well-lit area will allow you to see when the excess material has been removed. Using a sanding block is a very good idea, as you'll get a nice flat finish. Take your time with this - don't work the block hard, just let it remove material at its own pace.
- Clean thoroughly again.
- Check smoothness. You can do this using the back of your finger (so long as the stanchion is clean & dry), a paper towel, or ideally an old pair of tights. If you feel any roughness, keep polishing.
- (Optional) Use a metal polish to get the surface absolutely perfect. I've used Brasso, but probably any of them will do. You might get a good enough finish with the wet & dry. Clean again afterwards.
- Once you're happy everything is smooth, lube your stanchions. If
you've been riding a long time with scratches (and no other damage), you might want to think about servicing your forks.
Done! Now go ride your bike.
I tried using 2-part epoxy (Araldite standard), but it tended to peel, and stay raised up very high over the scratches. I've found nail varnish top coat (Mavala brand) to be quite suitable, but you will want to let it cure for 24 hours before attempting to sand it. If you're not sure, ask the nice people in the cosmetics store for the hardest nail varnish they've got.