Proper sanding procedures to check for crack

[COVRTDESIGN]

I have my Trek Boone frame that snapped a hanger and threw the derailleur into the chainstay. The chainstay did not fail and it seems to pass the tap test, but as an ex F16 crew chief, that doesn't fly with me.

What would be the best sanding practice and grits work best to get through the paint and give me the ability to look at the carbon to take a look? I would like to do this now to see if I can still ride it or at least put it on the Kickr this winter. I plan one getting it resprayed anyways in the spring/summer, so I do not mind having a square of no paint on the frame.

I am a certified A&P mechanic, F16 7 level Crew Chief, and SAE certified tech, so I am quite comfortable with my NDI and sanding skillsets, just wondering if any of my framemakers, painters, the pros, have some advice that they would like to share. Would rather gain more knowledge from the experts in this specific skills and try those skills rather than just waiting for someone else to do it.

Cheers!

[stefthehat]

Depends a little if you’re doing this via hand rubbing or using a small sander as the speed of the latter can affect things ,240 will cut through laquer/topcoat/primer v v quickly but you don’t want to start cutting into the carbon so either start with it and go onto 500 as you get close to exposing the carbon or just start with it to be safe ,iam assuming you’re using something like 3M wet/dry by hand ,you could localise your area with masking tape though 240 will def tear up the edges ,these grades can be adjusted but don’t go heavier than 240

[Mark Kelly]

I generally use a powerfile running 80 grit Norton Blaze belts to do the heavy removal and switch to 120 grit wet and dry by hand for the lighter stuff then 180 grit to finish. This is a lot quicker than the suggestion above and I have never had a carbon failure from using it.

Practice on something you can afford to scrap first, you will quickly find that the top coat, the undercoat and the actual composite matrix are pretty easy to spot. If the dust goes black you've gone too hard.

Check the archive for posts from Kevin Steegman of Polytube cycles (Data's Brother). He is truly an expert on this.