You may worry too much
I'm obsessive about scratches. Any scratches that I get are polished and primed before I proceed any further. While there has been incidents if cracking rudder skins, a cracked RV-4 engine mount, and exhaust systems cracking, has there been any failures to parts, as it pertains to RV's, caused by a stress riser? While I'm not advocating sloppy or careless work, I wonder if I'm too stressed(parden the pun) about insignificant scratches?
None of your examples are real scratch issues:
>Elevator cracks where mostly from early models using 0.025 skin and the angle stiffeners where much shorter, to ease getting the most aft rivet (closest to the trailing edge). The short stiffeners left a larger area of unsupported skin to flex across that last rivet hole, the place where the cracks started. Sure a scratch, burr, gouge or sharp notch makes it worse.
>Engine mounts? Welding not scratches or stress risers. More heat effected brittle zones. The corner weldement to mount the engine mount where just a less than optimal design. Again no stress riser issue, but yes it could contribute.
>Exhaust? Again welding, especially stainless steel can create material properties subject to brittle cracking. However like all metal under cyclic load and stress riser can make fatigue life shorter.
Scratches - A light scratch in the alclad (soft aluminum coating) is not going to cause a stress concentration, but there are other factors. The STRESS LEVEL of the part is critical to a stress related crack. Even with a deep scratch its possible the part will never crack, so location is critical. The loss of corrosion protection from the loss of the pure aluminum coating (clad). Corrosion can cause corrosion cracking.
The biggest factor is depth of crack, the deeper the bigger the riser. You are right to be worried about deep and sharp scratches, nicks and gouges, it can be crack starter. However most scratches are in the soft clad and not into the parent harder material.
How deep? If you can run you finger nail across it and catch it, than blend it out with some scotchbrite and prime just that spot, if you are not priming the whole surface. Don't go crazy about "Polishing", but make sure there are no sharp stress risers. Primer is only to get back the corrosion protection. Corrosion can be the start of a crack.
