Sealing of the wing is more important. Couple of types of sealant are useful - the stuff that comes in the tubes that you put on the seams before you join them, and then brushable seam sealant over the join once the wing is fitted and for all the exposed joins in the wing. Makes the difference between a job that lasts a year and one that lasts 20 years.
Paint the inside of the wing with anything, but make sure it is compatable with the paint you use for the outside if you are painting the inside first.