I would suggest making an 80% grease/20% chainsaw oil mix as the best
solution. 'Paint' same on with a good stiff brush as if it was a Weather Proof
paint. Make sure the rails are dry before application - if we do get a Summer,
let the rails get hot in the Sun for a day or two first, and apply this mix
when the rails are 'Sun hot'. They will be a bit greasy to handle, but they will
be well treated (the chainsaw oil prevents the grease from 'drying'). I use
this for some agricultural machinery that must be stored outside; it has
worked very well to date, and I am sure for U/G application it would be just as
good - if not better - since there is no 'weather' and your temp. is constant.
Hope this might be of use to you.