You could get some nice Brass domed nuts for £6, it'll stop the crap getting in and they also look nice as well. The only thing you might find (as I did) was that a few of them were too short and needed a washer.
http://www.ebay.co.uk/itm/M6-Brass-Dome-...1c3fa00e06
http://www.ebay.co.uk/itm/M6-Brass-Dome-...1c3fa00e06
Eagles may soar, but weasels don't get sucked into jet engines...