It sounds unnecessarily complicated. I would simply make a berm and plant in that. Choisya is not a fussy plant. Al

metal rusts. both the zinc galvanization and the vinyl coating are there to help inhibit and slow down that action. but it is a temporary fix... there are problems with both systems... 1) Zinc galvanization does rust and break down... it is a white powdering coating (zinc oxide). 2) the vinyl coating can crack, tear and rip... so that water can enter be trapped - rusting the steel wire from the inside. water, salts and acids can all help the rusting process along... ALL of which are present in a good healthy garden and compost pile. In welding we avoid zinc galvanization, because of the dangerous off gas that is created, but even that requires long term exposure and high quantities... do a topic search of: "zinc oxide poisoning" Vinyl arguably leaches potentially dangerous chemicals over time... do a topic search of: "vinyl leaching" sorry... too much information?

Hey, thanks for the replies! OK, well, fortunately, the rust is appearing only in isolated spots. At most it would take a gallon of primer to coat all of them. This is on a gable roof over a 20x50 foot workshop. So yes, it's a lot of roof, but I'm only planning on addressing the problem spots. The flaking isn't the zinc galvanizing - that doesn't flake off anyway. It just seems to wear away. The rusted areas seem to have a some history that is different from the rest of the roof. They look like they had a heavy application of some kind of primer that has cracked and lifted over the years. Underneath it's more discolored than rusted. I suspect these may have been areas that lacked galvanizing in the first place and were given extra primer to compensate. Anyway, most of these areas I chipped off the old primer/paint layers and got down to sound metal. The galv steel is also a heavier gage than what I've seen at the home improvement stores; I'm a bit reluctant to chuck it out when it just needs some touch up. I'm pretty sure it hasn't been painted for at least 25 years, and that's not bad. So I'd even consider blasting/primering/topcoating the entire roof, I just want to make sure it's the right products. I did some web research and found a fair amount of info since I posted. I don't think I saw the Sherwin Williams site yet, though. So far it appears to me that zinc-bearing primers are a bit complicated and not necessarily the best solution. Although I could see cold galvanizing the corroded areas (after wire brushing) and then using a suitable pimer. Also, so far, it appears that the most compatible primers with galv. is a latex based primer, similar to what Rustoleum offers. I am gradually losing my reluctance to use a latex-based primer on galvanized. I will visit the Sherwin Williams site and see what they have to offer, as well. Here is what I've learned so far about zinc-bearing primers, it seems that they fall into two categories: those with inorganic base, and those with organic base. The inorganics are basically cold-galvanizing but are also porous and do not provide much in the way of a barrier. The organics have a lower zinc content and a higher barrier performance, but ironically do not provide much in the way of galvanic protection. Then there there are the one-step epoxy systems that claim to dispense with the need for primer, but on an old weathered roof that probably never been clean enough, I'd probably go with a primer/top coat solution. Once I do get a good solution for the shop roof, the roof over the 2 car garage has totally shed its silvery paint. The underlying galv coating is still in great shape, no rust; I've been waiting for Mother Nature to finish off the paint there before I recoat it. Any recoating will probaly wait until next spring... the weather is a bit dodgy these days for full roof painting.

I doubt it is critical - paint if you wish. Even galvanized stock tanks don't last forever as a raised bed. btw, skip the gravel in the bottom of the tank. Contrary to popular belief, it does not improve drainage at all and it just serves to lift the perched water table that will exist in any container planting situation (as this is - with any bottom, it is not a raised bed) that does not have a well aerated, fast draining container potting soil as a filler. Just use landscape fabric or fine screening at the base to keep the soil contained and check out the 5-1-1 or gritty mix referenced here. If the leaching of zinc from the galvanizing process is a concern to you, you can line the tank with plastic, allowing provisions for drainage holes through the plastic as well. But countless years of livestock drinking from these tanks with no ill-effects from hazardous metal leachings may allay any serious concerns.