Here is a good dev answer to the question: https://ffg-forum-archive.entropicdreams.com/topic/108101-ffg-developer-answered-questions/?do=findComment&comment=2660152
Personally, I agree with the answer and wouldn’t do it unless we’re working with something like a personal deflector shield, in which case there’s no effect until you get to major damage (inoperable) or destroyed.
If you want more specifics, my suggestion is that Sunder only affects powered effects of armor, so shields, heating/cooling systems, maybe advanced optics, etc. This, I’d say, would function more like a component hit critical, and either you could handwave the cost or base it off of the attachment in question, if an attachment is, in fact, the item in question. If it’s a small component of a larger armor, you’d have determine the cost on a case-by-case basis.
The only thing I am aware of that references damaged armor or repairs to armor is the Self-Repair Systems attachment, and it seems like an anomaly to me, something that perhaps should not exist.