West System with chopped carbon.
Chopped carbon be prepared for a learning curve. Use too thin of epoxy and it just comes out with furballs and may not be distributed evenly. Too thick an epoxy and you sure has heck am not going to be able to inject the mess anywhere. I have my doubts about the additive strength of randomly oriented fibers. If there is too much chopped stuff in there, I believe the bond will be substantially weakened. I suspect there may be a "narrow" window where a given amount of chopped stuff adds strength. Too little and added work for no gain. Add too much and the bond is weaker.
Where one sees the most strength is where folks laminate fiberglass, or kevlar socks or carbon fiber mat in layers and rotate the orientation of the fibers in each layer. Do vacuum bagging and holee molee that is strong stuff.
I've injected "neat" Proline 4500 and Cotronics 4525B and 4525IP without trouble, without chopped fibers and am satisifed with the results. It is easy to get into trouble if working in tight spaces with a small diameter syringe and tubing if injecting CF gunk.
Get a clog and you are in deep doggy do do. Can lose your whole batch. That's why I only inject the "expensive" stuff neat. If you can use a large diameter syringe and tubing you can use more viscous "stuff".
If you are going to mix CF with your epoxy, try on EXternal fillets first as you will have the room to work with it and reshape it. Better yet, do small practice batches and slather it on "practice pieces" of plywood and see how it turns out.
Actually, I think surface prep is more important than whatever anyone can add to the epoxy. Clean with solvent. Use as course a grit of sandpaper you can get away with on the surfaces to increase the surface area of adhesion and clean again. Then go for it.
I used to use PowerPoxy Weld (out of production, NOT J&B Weld different stuff) and would hit it with a heat gun after 5 minutes or so. It would re-liquify for a short period of time and that would allow it to REALLY soak in. Of course it
would also accelerate the curing so it would start to set up quicker. I discovered after I did an autopsy on a crashed irrepairable fincan, I could NOT snap a fin fillet off of the plywood fin after I chopped it out of a cardboard rocket body.
I had to go at it with a screw driver to pry the bead off the plywood. Now that's adhesion!
Now some epoxies you hit with a heat gun and they will just set up quick and not re-liquify. I discovered Proline 4500 will reliquify so I am very happy I can use that technique with that product. Heat on 4525 if used has to be tried early on after application otherwise it will just accelerate the setup without having liquifaction first.
The ideal fillet adhesive is one that goes where you put it, stays where you want it and doesn't stay "runny" for hours and hours.
The PowerPoxy I could pull the tape after 15 to 20 minutes and if the tape interface bead "wasn't pretty" I could usually hit it a second time with the heat gun and get it to reflow and look nicer. Kurt