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So far, I've only seen other questions around here about galvanized pipe or duct, so here's a new one for you all!

I'm trying to patch up holes rusted through a telescope observatory's dome. The panels themselves are, according to documentation, Galvalume; the rest of the structure, g-90-grade galvanized steel.
I'm not a welder nor do I expect the community college has funds to hire one for this job, though I understand that would be the most ideal fix. I also never heard back after asking Facilities if anyone on staff had repaired galvanized metal before.

So, I think this is gonna be a "DIY" job. My thought is to repair by sanding/grinding away the rust, applying some sort of metal filler putty, and then painting over with a galvanized-approved paint. The dome is incredibly difficult to disassemble (i.e. we need to rent a crane), so I'll have to do this in-situ... so, products need to be OK with high humidity and possible rain (though I can cover with a tarp) while curing/drying. This is in Hawaii, where there's equal part bright sunlight (read: intense UV) and rain exposure, salt breeze (one mile from ocean), and no chance of freezing.

Q.1 - what sort of filler putty to use? Permatex makes a 25909 "Liquid Metal Filler", which claims excellent adhesion to galvanized. 3M Bondo Gold also claims adhesion to galvanized. Weirdly, I emailed 3M and they stated Bondo "is cosmetic only, not an adhesive" and that they "do not manufacture/market a product to meet your needs". Hmm.

I figure that I'll need a fiberglass or polycarbonate patch to reinforce the inside of the larger holes (well, definitely the couple of 3-inch totally-rusted joints that might just need to get chopped out).

Q.2 - how large of a hole can I get away just filling in, before needing a reinforcing patch? Half-inch? There are two spots that are just pinholes right now, but they're directly overhead (the sliding shutter tracks have foam insulation that's above these points), but once I sand them down I expect to have to patch a larger area.

Q.3 - Is there some reason to not use a "cold galvanizing" spray-can paint to finish over these repair areas? I am looking at Rust-Oleum 1685380, which states to not use another primer or topcoat when used on galvanized. It says it is an epoxy ester product.

Thanks for your time and any past experience!

Edit: Just found I could add pictures. Here are a couple to help illustrate: The bad seams, from inside: The bad seams, from inside The bad seams, from outside: The bad seams, outside Pinholes, about 12 feet overhead: Pinholes, about 12 feet overhead The worst area, plus surface rust: The worst area, plus surface rust

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  • Fascinating question. A little out of the usual realm, but some people may be able to help. Particularly Harper - he seems to know quite a bit about heavy duty painting. Pictures would help a lot to give some context. – manassehkatz-Moving 2 Codidact Dec 1 '20 at 0:54
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    Galvalume® is (as the name implies) not simply galvanized (zinc-coated) - it's a aluminum-zinc alloy coating. I believe that will make repairs much more difficult than straight galvanized. Incidentally, welding is probably not ideal as a repair method, anyway. – Ecnerwal Dec 1 '20 at 0:56
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    Too bad nobody thought to maintain this structure with some paint before it rusted through.... – Kyle B Dec 1 '20 at 5:15
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    Do you have a local Technical College? I'm sure they've got a welding instructor or two who would love to give an advanced student some supervised real-world experience! They'd probably even do it for free (or maybe for cost of materials). – FreeMan Dec 1 '20 at 13:27
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    The galvalume is the same coating used on some mufflers I have welded through it it is not as nasty as zinc coated galvanized metal. Rust is a cancer to sheet metal your first job would be to cut it out or wire brush it out and spray paint with a zinc rich paint. If needed you could use a body filler or weld it up. It could be welded the channel connecting the sheets will be the issue there and why I would probably use a filler and then paint it so it doesn’t show as bad. But a wire wheel and zinc rich paint are your best starting options that may be all that’s needed. Try the wire wheel first – Ed Beal Dec 1 '20 at 20:02
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If the rust is superficial and not has not corroded through the roof panels a light sanding with 60-100 grit sandpaper on a sander will remove the most of the rust down to shiny metal. What I like to do next is to apply a rust treatment such as Permetex Rust Treatment (found here

1: https://www.amazon.com/Permatex-81849-Treatment-10-25-Aerosol/dp/B000BKC25K/ref=sr_1_7?crid=1WIHUOS5UN97Y&dchild=1&keywords=rust%20treatment%20for%20car&qid=1607738762&sprefix=rust%20treatment%2Caps%2C213&sr=8-7). This will change the rust to a black inert polymer primer and prevent it from spreading. Once it is dry (black) it can be painted as usual. I'd recommend any enamel alkyd type paint specifically for exterior metal. Rustoleum has an Industrial-type paint for tractors and machinery (found here) If the rusted metal is perforated or has through holes you will need an extra step to repair the damaged area. Start by sanding the rusted area as before. Remove the rust until you reach shiny metal again. Spray with rust converter as before. Now here's where you'll need some artistic skills. Depending on the profile (shape, plane, detail) of the damaged area you'll need to fit a patch of galvanized sheet metal , cut larger than the hole. You'll have to shape and bend the patch to the existing roof section. Allow an additional 1-2 inches of the new patch to extend past the hole. Mark along the new metal edge where you will drill holes for round head screws to secure patch to roof. Next you can apply a continuous bead of 100% silicone onto the outside edge of the hole. Lay the patch over and screw down with sheet metal screws. Apply protective coating to repaired area. This is a general answer to your question. There are many products and several ways to repair a metal roof. I've answered with the easiest and most durable way I've found. Your roof, I'm guessing is movable to open and close. Be wary of any repaired area that may interfere with the moving roof.

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It looks like water ( rain or condensation) is being held in the bottom channel. I would look at drilling holes so that it drains. Being Hawaii there is likely salt in the water; you have more than 2 strikes against you . Galvalume is zine with some aluminum to give it a little more protective voltage /emf. It will handle pretty much like regular galvanized. I would make repairs two steps ; treat rust ,then look into patches. Grind to remove most rust, treat with conversion coating ( whatever the name ,they contain phosphate ). Then put on your zinc rich paint ,( don't worry about the brand, non are industrial 3 component zinc rich primers ). Then evaluate if you need patches ; fiberglas plus polyester is most common . This is a stop-gap job until a good resolution is developed. I could give you names of industrial coatings like Carboline and Napco but this is too small ( they like to sell 55 gallon drums) for them and some may be out of business since I retired.

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  • As @ Lee Sam implies , when they get serious look at ferritic 13 Cr stainless as a replacement , probably the lowest cost up-grade – blacksmith37 Dec 13 '20 at 2:09
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There are rules that do not allow you “trying” various repairs with public funds.

The rules require institutions to hire trained professionals, prepare construction documents (plans and specs), solicit bids and then observe construction to insure the work is done correctly. This is to keep people from spending public funds on construction without proper training.

There’s a rule that allows public institutions maintenance workers to do work on the buildings if it less than a certain amount. This amount varies at each institution but generally starts at about $50K.

This insures the design professional and the contractor have a certain level of expertise and insurance, in case something goes wrong. So if you add something that blows off and injures someone or they use the wrong paint and it creates a bigger problem, then they pay, not the institution.

Before you go trying to “help” your institution I’d check with your legal department.

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  • This is useful to think about, especially in the context of a roof. In this case, my supervisor has made it clear that this is being treated as a piece of equipment owned by our department, not as a piece of the institution infrastructure. As my job involves maintaining and repairing other major equipment in this department, supervisor felt comfortable asking me to research and suggest ways to repair this ourselves. – Dan C Dec 13 '20 at 23:22
  • And, as it turns out, the local welder we asked for an opinion suggested some of the same methods discussed in replies here for the small spots. He also suggested asking around for professional roofers who deal with galvanized for a final opinion before committing to anything... unwise :) So, here's to the pros in the end (as we should always ask for their advice, at least)! – Dan C Dec 13 '20 at 23:23

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