I suppose you don't have much to lose by trying it, but I'd have thought it was more likely to be gunk accumulating around the mechanism. But it's always safer and easier to tell your punters to wipe down the fabric - less chance of ending up being sued.
More likely the spring losing tension. Good idea pulling seat belts out and giving them a good disinfect at this time of year though, very prone to mould bacteria growing on the fabric in damp weather.
Well cleaning the belts in soapy water helped on my MX5 mk3 but didn't cure the problem completely. I am now going to try some polish as suggested. Just make sure the belts are dry before letting them rewind.
