I used to get rust on my the buckles of my old Karrimor hiking daypack. It was given to me from my uncle when I was about 7 so I've had it for about 20 years. What I tend to do is use fine sandpaper to get the rust off and then either apply some wax or a light dab of oil. I tried different things over the years which all seemed to work but maintenance is needed. For a while I taped the buckles with clear scotch tape which helped but wears out with frequent use.