I restored an original Kolb Twinstar several years ago. It spent its 1st life in central Florida. Most of the bolts looked at least as bad as the one on the left, in terms of corrosion. I had the original plans & parts list to work from, so I just ordered almost all the hardware in the list and replaced it. I don't know how tight your budget is, but if you're rebuilding a crashed a/c, I suspect that a complete set of hardware for a plane that size would cost well under US$100. The time saved in not cleaning/grading each bolt will be significant, but your time/money balance scale may be different from mine. The old bolts can go into a 'junk box' to be used for ground-bound projects. Until you get into really large bolts, AN hardware is surprisingly inexpensive; not much if any worse than grade 5 hardware store stuff. If you're like me and always tinkering on something aviation related, you may find that it pays, in both time saved and shipping costs, to order in increments of 5 or 10 *more* than what you need for a given project. I've got a pretty good collection of small-medium sized hardware, and I still find myself walking next door to my neighbor's house to find something I'm missing that he might have on hand. (He does the same with me.)
Anything with rust on it will continue to rust, and possibly worse, will accelerate corrosion of any aluminum it contacts.
The preferred AN hardware for contact with aluminum is plated with cadmium; not zinc. But...assuming a reputable kit maker, use the hardware they recommend. (Hopefully they didn't ship the kit with hardware store grade bolts...)
Charlie
