Currently, armor patches work by repairing the existing armor. While this works well, it has issues:
1. You can repair armor you shouldn't be able to repair
&
2. There's no tech limits
These could both be fixed with script, but in the longer term it would be nice to see a fleshing out of patches. A system that allows "layering" of armor has been suggested in the past. This, or the use of overlays, would allow patches to act as a separate segment on top of the existing armor, with their own resistance profile. This would allow for more armor patch items, with different strengths and weaknesses, as well as dealing with the simpler problem of 'fixing' magic neutronium power armor (as a hypothetical) with the space-based equivalent of duct-tape and some tinfoil.
Incidentally, this might also allow the use of "patches" as temporary extra armor in their own right. This might justify items designed specifically for that purpose, but would be pretty cool (especially if removing or replacing patches required dock services if the patch hasn't worn out completely first)....."hillbilly" armor is a pretty neat thing IRL.
This would make sense from a realism standpoint, but it would make them pretty useless after the early game, unless you could get ones with high level armor-equivalent resistances. And that would just be effectively allowing you to increase armor hp beyond the normal limits.
Personally, I like to imagine that armor damage is based on how close it is to being penetrated in any spot, so even when it's at low hp, most of the material is still present, allowing a sufficiently high-tech repair process to make it good as new.
I wouldn't object to repair consumables being limited to a certain tech level, as long as it's not too restrictive. Say, at least two up to level 10 and at least one up to level 11. Honestly, it's not worth carrying low level ones around to use on high level armor - they don't heal enough points to justify the inconvenience.
"This would allow for more armor patch items, with different strengths and weaknesses"
This implies levelling them up as you go along, you know.....
I think that the easiest solution would be to have armor patches be targeted to specific types of armor based on specific sets of attributes, and to just have them work less efficiently on types of armor with different/lesser sets of attributes, and maybe to not work at all on unrelated types of armor.
It could be that generic titanium/plasteel/orthosteel armor patches would repair their corresponding armor tiers up to 75%, with proprietary armor patches being needed to repair them past that.
The armor patches could also have charges so that you could choose between using up the entire patch or only using part of it for the repairs. The higher your repair skill, the fewer charges you waste when trying to repair armor.
The skill level could also give you an estimate for how much of an armor patch you need to use to repair the armor.
It could even become a selling point for some of the basic types of armor that they can be repaired without proprietary repair materials.