[tl;dr] It's not the mod's fault. (read the bottom)

Author:
I'm sure you've received this report before: After patching wall cracks, they return quite quickly. However not all of them return. What I suspect is happening is the Erosion Days timer is still 'ticking' and it thinks it's adding cracks to a new building.

Can you think of any work around that would remove the wall from the Erosion system? Some kind of change that would make the wall be flagged as 'already cracked' but maybe the crack is now invisible?

Since Erosion Days timer also exists on a server, I don't think you'd want to change the Erosion System itself, but rather change the wall crack and how the wall treats the erosion timer. - If it interests you of course.

- - Subs:
The cracks will keep appearing as long as you're before the max erosion speed day. So default is 100 days. After that, if its working as intended, you should be able to patch the wall and not have it come back. But that may not apply to MultiPlayer.