Its the games default method of repairs for vehicles and weapons and I did not change this behaviour for the mod. There are however other mods that will change this so effectiveness of continuously doing the same repair over and over does not weaken.

With this mod I added quite a few repair alternatives for each vehicle part. Once you become a skilled mechanic or welder the effectiveness of a repair significantly rises.
Eg: My main character is a level 7 mechanic and level 6 welder. If I use a blowtorch and sheet metal for doors the repair percent is nearly 95% effective. You can then repair the same part later with the same method but repair effectiveness will drop to 60ish%. Do it again and it'll drop further. Or use an alternative repair recipe for that initial higher repair percentage.

We are in the apocalypse and I do not think you would realistically have a door for example that is 100% with constant repairs. Its damaged and your basically patchworking it. Repairs get you by, for quite a long time too. I have a car I have been keeping and repairing but if I want it back to 100% pristine I would have to find either a new part in a box or a part thats been kept immaculate.

This is the compromise, balancing repair effectiveness but also meaning you will still want to keep a look out for pristine vehicle parts. Thats why its also key to have a skilled mechanic in your group, they can not only acquire the best parts by uninstalling them from found vehicles but can also significantly repair the cars you do have until new parts are found.

Summary..Look at repairs as a patchwork to keep the condition up, the more you need to repair it the weaker the part becomes. This will keep you on the search for other vehicle parts and will encourage you to stockpile parts after repairs become less effective.