110 rear X-member rust ?

Once the rust is there any protective layer must have gone. Treating and a good paint has to be the remedy, and Kurust and Hammerinte seems to be doing ok for me at the moment. I've also used POR-15 on the chassis which seems to be the mutts nuts, but beware - you can remove it from skin, but only with a scouring pad and a lot of elbow grease. If you've got rust on the surface where it's visible then there must be rust where it's not visible. I'd give a good blow with an airline to try and get the mud out from the box sections, followed by a good pressure wash and a dry week to dry out thoroughly. Then give it plenty of penetrate rust treatment, I used Dinitrol (from Frost) because it's in aerosols and I'm lazy, but there must be others.