I removed my running board strips today and found the steel bolt heads rusted away to almost nothing, the nuts were fine having had a good coat of Waxoil or similar. there was no corrosion underneath the strips and the holes in the wing show no signs of degradation at all. I'm going to try this suggestion and have ordered 3mm nylon nuts and bolts from Westfield, I'll fit them next week and let you know how I get on.