Sorry guys, but maybe in your South climate, nuts & bolts after 40-45 years are ok, but here in East-North part of Europe most of them are much worn out/rusted etc. & as for me all are trash, zinc plating will give them just some bling bling, but technically they are wasted. I personally broked around 20x.
If we talk about zinc plating of good condition / usable used parts for example: axles, struts, calipers, springs or cover plates - it is very good, because it is additional protection for metal oxydation process & in addition with powder coating, part will be rust free for much, much longer, than just raw metal painted with standard paint or even powder coated.
Working parts like brake discs etc. it is always better to replace with new one's. Ask yourself a simple question - when in your daily driver car start to have an issues or have worn out part, do you take it to for zinc plating or powder coating? probably not; - it is pretty obvious that you just replace defective part with new one, or at least for fully/professionally rebuild one.
Of course if you brake discs are like new or have almost fabric specs (thickness, flatness etc.) you can zinc plate it to avoid rust.