Jack,
Yes for galvanized you need a special primer or the paint will peel off in time. You mentioned Rustolem. I have an old can for aluminum and galvanized, it is Gray 8781 and is called aluminum primer and is latex like house paint. I have no idea if rustoleum still makes it, its about 12 or more years old. It primered a heat pump cover with no peel. You can expect about a year before rustolem paints will fade and get chalky, but this is a gate so you don't have to have automotive quality. For a moving object and metal in general i don't prefer a latex paint. It chips easily. You can also use cold galvanizing compound which is gray. Read the label, some/most of those can't be top coated with other paints because of peeling. There maybe a newer type out i don't know about. If you go with that, I would get a qt and and spray it on. A Spray can type has to be thinned so much to come out they are a weaker formula. If you don't have a spray gun, than a rattle can is your only choice. But i'm getting ahead you need rust treatment first. The best way to get rid of rust is to remove it. Because of the angles you won't be able to reach all the rust and remove it with a brush. Blasting is the way to get it all but since thats not an option a rust treatment is a good choice for your gates. I will answer your rust treatment question as best i can. Navel jelly is a very weak rust remover, there are much better acid types such as oxisolv. This acid removes rust and leaves zinc phosphate in its place which prevents rerusting for a time and serves as an etch for paint adhesion. It is automotive quality and about $25 a gallon. This is for removing rust and i don't think worth the effort, labor, and cost on your gates. The metal has to kept wet for 1 to 4 hours constantly but it will remove all rust. Navel jelly won't get it all. The Ospho Kendall mentioned is $20 at Ace hardware. This product is for converting rust, not removing it. It is supposed to chemically cause rust to change to iron phosphate, an inert material. That is supposed to stop rerusting once the Ospho is primered and painted. You must first wire brush all loose rust until you have only tight rust, then apply the Ospho. Ospho is a water thin coating of primer and metal etch treatment in one. Ospho leaves a coating which can be a clearish black to a white powder on heavy rust. You must scuff off the heavily powdered areas before you paint over it. Ospho recommends 2 coats on heavily rusted areas, so you must use some judgement when working with this product. I have used it once just recently on a rusty trailer ramp. It has a good reputation in the salt water shipping industry for refinishing ships. For galvanized metal, not as easy. You must protect the good areas from Ospho or it will damage the galvanized coating. You can carefully apply it only to the rusted areas. They are vague, stating only to use it on all the galvanized if appearance is not important. I don't know what that means. If you're repainting something, appearance is important to most. My guess is if you're only interested in stopping the gate from rusting away, then it doesn't matter if you cover the whole thing and it effects your topcoats look. ? It sounds like the other product you have is similar, but is more a paint type product. These are supposed to work in the same manner, by converting the surface rust to an inert state. They usually dry black in color and have pigment and feel like paint and are thicker than ospho. You top coat over them too, whether another primer is needed depends on the product. Extend is another brand of this rust converter type paint, another is Rustoleum Rust Reformer. Another expensive brand is Por-15. It is a thicker harder paint type product that is more like top coat and doesn't need a top coat except in sunlight. Designed for underbody rust on cars etc. I don't recommend this one for galvanized. Since you can't blast, here are some options For painting your gates. You can either use the rust treatment you have, or another and any needed primer only on the rusted areas , then use a primer for galvanized on the unrusted areas and top coat in the color of your choice. Another way to go would be to use Ospho on the rusted areas and the cold galvanizing compound over the rest if you wanted them to be grey. I don't think the cold galvanizing compound would stick well to the paint type rust converter. Just an opinion. I think it would stick to the Ospho. Again, just an opinion. Unless the gray color is a problem, I think the cold galvanizing would last longer and be more durable than a Rustoleum top coat over the rust converter you use. Especially if the only primer they offer is the latex. If so i would consider another brand if paint is your choice. Sorry for being so windy, galvanized is a special problem when rust is involved, you have two differant substrates needing differant primers you have to deal with and trying to explain how rust treatments work and how to use them is a deep subject.
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