Hi,
if it is an original paint job and the paint is patchy like a lot on them then painting over it looks good for awhile until the paint that hadn't lifted already decides to lift and ruins your new paint coat. Sanding back the gal is problematic because you will probably cut through the gal on the surface and then you don't have the gal protection.
Glass bead and or soda blasting can remove the paint without removing the gal so you retain the protection and get an overall better long lasting finish, assuming you do the prep and paint properly as well.
Terry