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On of my fork stanchions has scratches on the black coating. The scratch has sharp edges.

The Fork is a RockShox Pike RCT3.

I could try to fill the scratch with paint and polish or replace the crown with standpipes. The replacement CSU unit for the Pike costs 316€

enter image description here

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    So - what's your question? Are you asking how to touch up the paint, or how to stop it rusting? Looks to me like minor usage damage - you rode your bike past a thorn or stick , or maybe it hit the ground. Some clear nail varnish might be all you should apply, or even nothing at all. – Criggie Sep 17 '16 at 19:47
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    What you need to do depends on the scratch (and your question too). If there is a burr (sharp barb sticking out) this will tear the seals. If there is just a scratch this may lead to oil and air leakage. Burrs need to be filed off and scratches filled to keep the fork working but a new CSU is the only fix. Plenty of instructions online on how to fix. – DWGKNZ Sep 19 '16 at 1:19
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    Wait - are you saying this scratch is on the inner part of the leg, that has to form a good seal? Cos it looks like the outer part of the suspension from your photo. Please clarify. – Criggie Sep 20 '16 at 4:30
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    @Criggie This is the upper fork assembly. Since a few years ago, Rockshox stanchions are glossy black. You can see the travel gauge printed on it, which is used to set the sag. The lower legs are joined by the brace that has "Pike" printed on it. – Will Vousden Sep 20 '16 at 10:16
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    @vaious comments: Just to be clear, scratches on stanchions is a leading cause of forks getting completely trashed. Sad but true. – Nathan Knutson Sep 20 '16 at 17:41
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It really depends how deep the scratch is. If it's deep enough, then a new CSU (crown-steerer unit) is the only sure-fire fix.

However, here are some steps I've taken in the past, which have given me some success:

  1. Sand the scratch down to remove the burrs. You can use super-fine wet & dry paper for this; something like 2000 grit should do the job without damaging the rest of the surface.
  2. Clean the scratch and the surrounding area thoroughly with isopropyl alcohol, removing any debris from the scratch itself.
  3. Fill the scratch with some high-quality epoxy. I've used J-B Weld in the past with some success. Some people have reported success with super glue or nail varnish, too.
  4. Wet a razor blade (or something similar) with water and use it to scrape off the excess. You want it to be as smooth and flat as possible.

Your mileage may vary here; if the filling comes off after riding, you might be looking at a new CSU if you want a complete fix. The fork will still work, of course, but a gap will allow dirt into the lower legs, which will effectively shorten the service interval on the fork.

I've successfully repaired a nick in a Fox RP2 rear shock like this, and it's still going strong today. However, it didn't work so well on the stanchions of my TALAS 32, since the filling wouldn't stick for more than a couple of rides.

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    If you're going to downvote, please explain what's wrong with the answer! – Will Vousden Sep 20 '16 at 12:26
  • Just dab on some nail polish!! – Daniel R Hicks Apr 17 at 16:08
  • I've done this before on a fairly basic motorbike fork that had quite a nasty stone chip on it, so the technical aspect works fine. I would imagine though that you'll need to be really meticulous with the surface as MTB components are a lot smaller and much higher performing. – el_oso Apr 18 at 10:11
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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:

  1. A filler (see note below). I use nail varnish top coat.
  2. A spoon. Don't use the good silverware.
  3. Fine grit wet & dry paper. 1500 grit is good.
  4. A sanding block.
  5. A cleaning solvent that leaves no residue (isopropyl alcohol, or brake or contact cleaner are fine).

Steps

  1. 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.
  2. 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.
  3. Clean the stanchion again as before. You want the scratched area to be very clean.
  4. 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 scratch.
  5. 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.
  6. 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.
  7. Clean thoroughly again.
  8. 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.
  9. (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.
  10. 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.

On fillers

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.

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Scratched stantions are no big deal as long as you smooth the edges and the scratch isn't deep you will be fine. I scratched my stantion on my reacon gold and feels fine. It's more aesthetic than practical. Just ride!

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If you just want to hide the chip a little just touch it a couple of times with a black sharpie pen. If that's not good enough, get a suitable vehicle touch-up colour & the lacquer, and do it properly.


Ah sorry.. I didn't realise the image was of the inner fork leg.

The only permanent option here apart from a new part is to remove the inner fork leg and take it to an engineering workshop.. A good shop should be able to grind the leg down in the damaged area. Then either fill it and machine it back to size, or spray weld it which is a different process to fill the damaged area, then again machine it back to original diameter. Filling by welding can be done with most materials in one way or another.

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    Sadly this is more than just cosmetic damage. A scratched stanchion can damage the seals and cause oil leakage and contamination. – Will Vousden Sep 20 '16 at 10:17

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