What to paint a rusting metal roof with

Question

I have a bump out in front of the house with a sloped metal roof on it that is rusting. I want to fix this before it becomes a problem but I can not find reliable advice for my particular situation. The roof is sloped, painted white and made up of many 1.5 x 3 foot flat metal sheets whose seams are covered in some substance, not welded, that looks like thickly laid caulk but not soft like caulk. The roof is starting to rust bad, maybe 40% is now rusted, and I don't know if I can just sand the rust off without disturbing whatever the seams are covered with or if removing the rust is even necessary.

My question is, what should I use to "repaint" the roof and how should I prep the roof for said "paint"?

Edit 1 here is a pic of the roof:

Answer 1

Short answer: go to the hardware store, buy whatever, paint it down and watch it fail in a few years. What you do at that point depends on what you used.

Or you could consider over-roofing it with EPDM rubber, using the rubber-cement-like glue to stick it down (paint both surfaces and join them). The easy made-for-wood glue won't work. Careful pricing this solution, the rubber is cheap enough, the cost is in the adhesives.

Long answer: you're not going to like it.

Galvanized steel is quite a good roof, until it's not. Then you want to deal with it swiftly. You have a roof which is also shaped to provide a roof drain not on the edge. Hard to replicate.

NASA has a lot of steel structures. They throw serious money into corrosion research, and they have a web site just for that. http://corrosion.ksc.nasa.gov/

I spent hours reading it, and sure enough, their advice conforms to my experience fighting the same fights. To summarize:

• Your choice of paint is not as important as your surface prep.

• That said, better paint is better.

• Chemical rust "converters" don't do much.

• Out of heavy disc sanding, abrasive-disc grinding, needle scaling and media blast, media blast to near-white metal (SSPC.SP10) wins, needle scaling is a distant second, and the others aren't even worth the trouble.

For removing rust, I decide how serious I am. Either I'm willing to break out the gear and do full-on media blast to SSPC.SP10... or I'm not. If I'm not, then I do what I can with hand wirebrushing and palm sanders, heat it over 212F with a hair dryer (to remove all water), wipedown, and then... umm... I haven't found much that works. To my surprise, West System epoxy used as a paint has held up 3 years so far. I used it because I was using fiberglass mat to bridge some holes. However, it needs to be primed and topcoated, because epoxy by itself is vulnerable to UV light and will turn to swiss cheese. I am also experimenting with plain ole Rustoleum 7769 rusty metal primer, then a much better primer and paint, just to see if it'll perform as advertised. I haven't actually tried cold galvanizing compound over rusty metal. Remember, this is a difficult problem to solve, with no good answers.

Cold galvanizing compound is basically paint, but with 90% zinc instead of the usual fillers and colorants. The can is so heavy that a gallon will break a gallon paint shaker, and it must be agitated a lot or the zinc will fall to the bottom of the can/cup. Zinc is what actual galvanizing is. After a year or two it weathers to look a lot like real galvanizing.

West System is wonderful stuff to have in your home workshop with 100 uses. Bit of an up-front cost but you'll never buy epoxy or bondo again, and you can now do fiberglass layup.

If I am serious enough to media blast, then I would lay several layers of masking tape (or duct tape over masking tape) to protect seams I didn't want to damage. Blast, wipedown with solvent, and paint either the cold galvanizing compound and done... or if possible zinc-chromate primer (Rustoleum even makes one, in the 7400 line)... followed by the proper primer for my topcoat of choice. I would expect 10-30 years out of that, depending on the quality of the topcoat, and if it's caught early, unlimited extensions simply by repainting.

Now as far as elastomeric rubber coatings or plain old "tar", it will gap you for a year or three. When it starts to leak/fail, you can do it again 1-2 more times. After that, 3-9 years out, it becomes too thick to seal - with cracks and separations being too big for further coats to bridge across. Now, your options are unpleasant. You can painstakingly scrape the stuff off, but I find it not worth it. Or, you can remove the roof and start over at that point. So tar-like roof coatings are a dead-end solution.

If you own this house, resale value is a factor. A cheesy repair will be caught by a homebuyer's inspector, and he will tell them to price a roof replacement into their offer.

Answer 2

First: yes you need to remove all the rust before repainting, or the paint will just flake off as the rust continues.

Next: you need to make sure that there's some "body" left after initial sanding/scraping of rust, or you might as well replace the sheets. You also need to verify that all rust is on the top, i.e. there's no leaking and bottom-side rust (or you'll need to fix the leaks).

Assuming all that went "ok," then sand & scrape all the loose rust off, and do your best to sand off the rest. Paint all exposed areas (with rust pits or not) with a "rust reducer" paint, available at any hardware store. Typically these go on white and cure black; they both reverse the oxidation (rust turned back into iron) and provide a good surface for painting. Then put down whatever color of metal-compatible paint you want.

Answer 3

Wire brush the whole area to knock off as much loose material and surface rust as you can, don't worry about the seams because you are going to waterproof them next.

Purchase 1 gal. of good quality white elastomeric roof coating and a roll of fiberglass mesh tape (you can even use fiberglass drywall tape if you have some or if it's cheaper than specialty "roof tape" because it is the same stuff anyway).

Sweep the area well and wipe with a damp cloth to remove all dirt.

Using a wide paint brush that you will never be able to use for actual painting again (not a throw away "chip brush", they are lame), paint a wide thick layer of roof coating at each seam; embedding mesh tape and applying another coat of cement on top immediately. You will have to work in sections, cutting the tape to length. You will get partially covered in the coating, you can wash up later.

Let that dry for the night and the next day use the brush to apply a nice coat of the elastomeric over the whole area. Repeat with a second coat the next day.

You will be good to go for years. If some rust bleeds through a decade from now (it might) you may need to throw another coat on then.