Follow the directions on the bottle. Make sure your coating system is mutually compatible and proper for the task at hand. Start everything with a thorough scrubbing with degreaser and a freshwater rinse. Then you must remove all scale and loose material including all the way to the bottom of any pits. Enrust needs light surface rust to react and bond. That means after you’ve sanded or feathered the interface, reapply fresh water to the area you’ve prepped and allow surface rust to form. Once thoroughly dry, generously apply well mixed (that has not expired or reacted) Enrust with a brush (not a roller) at 90 degree angles with about 20 minutes between coats. Allow to cure 24 hours. If you’re not satisfied with the appearance of the coat after 24 hours, reapply Enrust and allow it to cure again. Repeat until all traces of surface rust have vanished and an airtight epoxy is all that’s left exposed.
Once this state has been achieved, it can be left that way for some time and even exposed to saltwater. All that is required is to scrub the area with soap and water, rinse it clean and remove all residues, and reapply Enrust when dry. Once you begin priming you are committed to continuing though the final topcoat layer. Inadequately topcoated primer that has been exposed to salt spray must be completely removed and the process restarted. At no point in this process is anything resembling chemprime or Ospho to be utilized. I find that 2 coats of primer and 2-3 coat of paint applied under these circumstances are quite durable.
This process assumes that white blasting and the proper application of complete coating systems is not an option.