Wow, that's going to be an undertaking for sure........I'm not sure I've got *the* correct answer for you, but I do have some thoughts:

1) You may try calling Rustoleum, if you want to use their product, and see what they have to say.

2) I've got a book at work (can get it Monday) that is full of products for coating water tanks and equipment at water/wastewater treatment plants.....I have to imagine this stuff is better suited for your needs. I'll repost with more info for you.

3) How big is the roof? Have you given thought to cost/time/labor of painting at every 5 (or correct number) year interval versus replacing the roof? I have no idea the costs of either, so it's just a thought. I know the newer colored metal roofs often come with 20-yr warrantees.

4) I wonder if it's the primer that's flaking off, or galvanizing? I'd be worried that even if you recoat, you'd still have flaking issues underneath.

Best of luck, keep us posted, and I'll follow up with that material.

David

I agree with David regarding new roof v. painting. Whatever you decide to paint with, the manu. will tell you all loose material must be removed. This includes rust that can be wirebrushed off, which can be a daunting task if your roof is very large. NAPA auto parts sells a product I have used on old plow discs I use to make fountains. I have to remove all loose rust with a wire brush and put on two coats. It dries clear and so far has worked well even under water. The downside is it only comes in quarts and is about $11/quart.
Hope I've helped,
Mike A

Hi spam

In your web address bar type in sherwin williams; then go to welcome to sherwin williams; use the coating system wizard; they have a zinc clad II H.S. primer with 83% zinc by weight, solvent base paint. You can also contact them for a paint for use on your roof. I would think that rust-oleum could provide the same service.

Have you thought about using a GOOD high pressure washer to clean the loose rust off the roof.
good luck bill

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 came across a product called "Rust Bullet" that claims to convert the rust to an inert substance that could then be painted. This might help. Does anyone have any experience with this product? Here is their URL
http://www.rustbullet.com/productcart/pc/viewCart.asp

phophoric acid is sold in a variety of trade names and i think is the best product to rescue rusted steel. it does convert the rust to some black oxide or something, which is a good base for a primer and then paint.
the chemical conversion is much more relaible, and easier, than trying to wire brush all the rust off(which is impossible) . I have never found a primer that actually sticks to steel that had been rusted.
buy this at adavnce auto stores for about $6 for a quart

I have an 10 x 16 shed with a galvanized metal roof. Shed is probably 35 years old and has lived its life on the Texas Gulf Coast - 500 yards from salt water - so this is about as tough an environment for a metal roof as you cam imagine. Four years ago I noticed that the galvanized metal roof had a lot of rust - about 25% coverage I would estimate. Most was surface rust, but I had some pitting as well. I hit the roof with a wire brush cup on a right angle grinder to remove all the loose rust. I then coated the roof with "Ospho" which is a brand name for phosphoric acid. It is advertised as a rust killer that converts rust to a black protective coating which can then be painted. (If you google "Ospho" you will get lots of information on the product - about $27/gallon from Amazon to cover 600 sq ft according to the ad.) I then painted the roof with an aluminum based roof coating from Home Depot - like thick silver paint. So far - no rust, no flaking and no peeling. Someone above mentioned "Rust Bullet." I have used a number of similar products - Black Star Rust Converter, Restore, Corroseal, etc. Many of these use an acid (phosphoric acid, tannic acid, oxalic acid, gallic acid, etc. in a latex paint base. The acid converts the rust and the latex base forms the coating. I have had mixed results with these products on steel ibeams in my environment exposed to salt air. Even with a quality oil based paint topcoat, I did not get lasting rust protection - probably because I did not prep or apply the products correctly. The Ospho/metal roof paint solution has worked well on my shed roof though.

The reason why galvanizing is so hard to paint is the same reason why it resists corrosion: the zinc forms a hard, oxide 'skin' very soon after coming out of the zinc pot. The zinc will keep resisting rust until it is 'used up' and that's when the rust appears. This zinc oxide coating also interferes with traditional paints, as they cannot 'key' to the base metal, rather just the oxide skin - and it flakes off. Ospho is a good choice, as the phosphoric acid converts rust (iron oxide) to iron phosphate and also 'keys' the zinc. "GalvaGrip" is a good paint for intact galvanizing, but an acid wash and proper prep is essential.

Cheers