Is there a reason I can't find grade 5 hot-dip galvanized nuts? I'm only finding zinc plated grade 5 nuts.
SAE J429 Grade 5 falls under the SAE International specification, formerly known as the Society of Automotive Engineers. While similar in function to ASTM, it is the scope of their standards that is different. SAE has a more narrowly focused range of standards. Original Equipment Manufacturers (OEMs) tend to use more bolts manufactured to SAE standards than ASTM specifications.
Galvanizing is used to protect bolts from the elements. Grade 5 bolts aren’t usually supplied in a galvanized finish because their application normally does not call for it. Since the bolts are rarely galvanized, there is no need for the nuts to be either. Since galvanized nuts are rarely used, they are not readily available in the marketplace. Portland Bolt often supplies ASTM nuts that would match similar chemistry and mechanical properties that are readily available. Plain finish SAE J995 Grade 5 nuts can be run through a galvanizing line, but the nuts will then need to be tapped oversize to accommodate the thickness of the zinc on the threads of the bolts which is both labor-intensive and expensive. If possible, consider using a galvanized ASTM high strength nut in lieu of an SAE grade 5 nut when used with a galvanized Grade 5 bolt.