Be sure that all loose paint is removed. Otherwise, the problem will occur again after another painting. Don't take shortcuts - correct the problem now by doing the job right. If you are using latex-based paint, clean the sanded area with a good grade of solvent. Apply a heavy coat of the solvent and allow it to evaporate.
Special solvents are available for treating galvanized metal.
After the solvent has evaporated, apply the latex paint directly to the bare galvanized area. For large areas, finish the job with two top coats. If you are using an oil-based paint, prime the sanded areas with a good grade of metal primer. After the primer has dried, apply one coat of a good grade of metal paint. Finish the job with at least one coat of a good-quality house paint.