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Best Paint for Galvalume?
Dear MRA ... I am the happy owner of a ~6-year old home with a "Galvalume" rolled steel metal roof. I'm planning ahead for the day when I'll need to apply an additional coat of protective paint and have begun my research accordingly. In my humble opinion, the supplier of this roofing does not fully understand their product. Their advice to me was simply to use a Rust-oleum product. Now, this may not be bad paint, and may even be the perfect match in the end, but my gut feel was that they simply did/do not understand the "chemistry" of their product. For example, I asked them if this roofing's chemistry was such that the process of oxidation was creating a beneficial "coating" that I should take care NOT to remove in the pre-painting preparation process. They didn't know what I was talking about, nor did they offer to investigate. This did not leave me with a good feeling, as you can imagine. Perhaps I should've done my painting research at the same time that I was shopping for a metal roofing solution, over five years ago. Mind you, I am totally satisfied with the performance of this roof. It seems to be holding up very well, and I feel that I made a good choice. But finding a competent authority on the follow-on painting matter has been challenging so far.
And so I have turned to the internet for other opinions. Any guidance on selecting the best paint, as well as on the preparation/application process, is very much appreciated!

Sincerely ... Mark Diehl - Cumberland, MD
Mark, do you know what sort of coating / paint system is on the roof now?
Hi, Todd. I did some web research and found the best info at http://www.steelroofing.com/trade_specs_data.htm (excerpts listed below). Although, after all this research, I can find no site that lists that actual type of paint used on a factory Galvalume finish (such as is used on my 4:12 slope roof). I am guessing that my roof is of the "AZ50" variety, as per the tech data below. Perhaps the factory paint process is a trade secret? One promising claimed fact that I did find is that this roof may not require field painting for at least 20 years. Although, the supplier of my roof suggested painting it after 5 years (this same supplier advised me to use Rustoleum). I am again not left with a warm and fuzzy feeling about their advice. Lucky for me, I used aluminum chimney flashing! The supplier made no mention of the risk associated with using copper flashing.

In order for you to best answer my question, do you think that it would be prudent if I were to contact the manufacturer directly, so as to obtain the specific factory paint information? I can't seem to derive this from the web. Once again, I thank you for your time and expertise ... Mark

Galvalume Tech Data Follows (from steelroofing.com web site) ...

Applicable Standards: Galvalume sheet is produced according to ASTM Specification A792/A792M-97a "Steel Sheet, 55% Aluminum-Zinc Alloy-Coated by the Hot-Dip Process." It is supplied in commercial, lock-forming and structural qualities. Each grade is available in AZ50 (AZ150-metric), AZ55 (AZ165-metric), and AZ60 (AZ180-metric), coating weights. AZ50 or 0.50 oz/sq. ft. (150 g/sq. m.) coated Galvalume sheet is generally used for prepainted architectural Galvalume SSR and AZ55 or 0.55 oz/sq. ft. (165 g/sq. m.) for unpainted structural Galvalume SSR.

Bare and prepainted Galvalume sheet have also been incorporated guide specifications for metal building systems and components by the U.S. Army Corps of Engineers. Some typical specifications are: CEGS-07413, "Metal Roofing and Siding Plain"; CEGS-07415, "Metal Roofing and Siding, Factory Color Finish"; CEGS-07416, "Metal Buildings"; and CEGS-13120, "Standing Seam Metal Roof System." The Department of Navy also recognizes prepainted Galvalume sheet in guide specification NFGS-13121.

A survey of 82 unpainted Galvalume low slope (1:12 slope or less) roofs up to 22 years old in the Eastern-half of the U.S. reconfirms the prediction from the R&D tests that Galvalume roofs will last 30+ years. The Galvalume steel roofs are in excellent condition and easily meet their 20-year performance warranty. After 20+ years the 55% Al-Zn coating is intact and protecting the steel substrate from corrosion. Based on their appearance, most roofs should last 30 to 40 years, before requiring major maintenance, such as field painting.

+++ End of message +++
Todd ... I also found the following info (listed below) on the steelroofing.com web site, albeit it of marginal usefulness (p.s. my roof is not the standing-seam variety. My roof is overlapping panels, fastened to purlins using hex-head wood screws).

Web site paint info:
Galvalume sheet for architectural SSR systems is typically supplied with a baked-on paint finish. Various types of paints are applied in a wide range of colors and finished on a continuous paint line. Processing on the paint line includes cleaning, pretreating, priming, painting and baking of the paint all in one continuous process.

Two coats of paint are normally applied to both sides of the Galvalume sheet: a corrosion inhibitive primer and a top coat. The primer is applied at a thickness specified by the paint manufacturer (typically about 0.25 mil). The top coat thickness varies depending on paint type and end use. Paint types include polyesters, silicone polyester, fluorocarbons and plastisols.

+++ End of messge +++
Posted Oct. 4, 2004:
Todd, I also found some paint-related information about Galvalume on Bethlehem Steel's web site http://www.intlsteel.com/PDFs/products/pepaintgal.pdf . While they mention paint manufacturers by name (e.g., Sherwin Williams, PPG, etc.), I have yet to find on the 'Net any discussion or offering of specific paint "facts" (i.e., the science of paint as it applies to metal surfaces). Geez, what's a poor homeowner to do, Tom? Surely there is someone out there in the world who has some hard answers about painting Galvalume metal roofing? Cheers ... Mark
I have a 7 year old home with natural (silver) galvalume on an 8:12 pitch roof. I love the metal and the look, but in the past couple years, we have 3-4 panels that have remained fairly shiny and the remainder of the panels look as though they have a film over them. Is this common? Or is this a material flaw? Regardless, is there a way to clean the metal safely or is it best to just leave it be?
I suspect this has something to do with the way the acrylic coating has weathered. I would guess that over time things will even out and all match. I would not do anything to it.
Depending on the condition of the galvalume, There are several ways to go about it. Acrylics are bad. Pourous, water goes through them and actually speeds up the rate of rust formation. The protective layer(oxidized zinc) should be removed and a paint that has some cathodic protection would be best. It is all about transfer of electrons. There are aluminum based paints that provide some cathodic protection. There are zinc rich primers that can be applied but are expensive ($100 +/gal) that really do the trick. If there is a little rust, a rust neutralizer should be used first then the zinc rich primer (spot priming the rust), the a coaating with cathodic protection. This system is installed and with get extend a roof life for 10-20 years with an easy re-apply of the coating after 10 years
Good for you on bringing up the question. We made a home out of a quanset hut. Love it. I would find out what the military did to take care of that problem. Since they have delt with this type of metal covering longest.I too have wondered about the chemistry in Galvalume. Try Steel Masters
I have to disagree with the comment Acrylics are bad. When applied over Galvanume they do not increase the rate of corrosion and are benificial compared to using a coating that is totally impervious to vapour transfer, overlaying with a coating that suffocates the system can result in accelerated corrosion of the cut egdes. Coil coaters won't supply a plastsol coating over Galvalume for corrosive environments as the edge corrosion quickly eats away at the edge of the steel resulting in the coating blistering, unless the Galvalume surface is allowed to dry and the natural oxides form the rate of corrosion will accerate.
Galvalume works because of the stability of the aluminium oxide that forms on the surface, the oxide is very stable in most environments and creates a barrier that effectively eliminates further oxidation, the zinc is trapped in pockets within the Aluminium continuos layer and only really comes into play at cut edges where it provides sacrificial protection to any exposed steel.
If you have a fluorocarbon (kynar) then you can be assured that the coating will last a significant time 20+ years, Silicon Polyesters are OK but there have been some disarsters, coil applied acrylics are in between.Sometimes failures are caused by the types of primers used in combination with the top coats. These typically show up quite early.

Galvalume roofs if installed correctly should last well over 30 years. Over coating uncoated Galvalume, make sure the surface is clean and dry (detergent wash and waster blast at low pressure, any areas where corrosion has occured should be treated with a zinc rich primer, you can then prime with as primer which is suitable for metal roofs - there are water based acrylic options avaliable. Then top coat with a good quality water based acrylic. Overcoating coil coated finishes, you need to ensure the surfaces are clean and dry, prime if there is corrosion but idf the finish is still in reasonable condition usually a good quality acrylic will be enough.
http://www.colorbond.com/index.cfm?objectid=17A44023-9350-961C-F5971686A4C3C3A2 Bluescope steel recommendations for recoating.

If you check out sites like Bluescopesteel.com.au ( they own the licnece to Galvalume world wideThe technology is licensed internationally by BIEC International Inc., a BlueScope Steel subsidiary.) and Decra.com they make pressed metal tile roofing out of Galvalume, they use acrylics to adhere the granules to the tiles. The coating on the tiles last longer than 30 years.
Thanks for the detailed and helpful info.
I have a 6 year old Galvalume v-crimp roof that I painted white for energy efficiency purposes. I scrubbed it with TSP, rinsed well then applied one coat of Ace hardware galvanized latex primer and followed with one coat of Ben Moore exterior white house paint. Still looks great 5 years later and really cut the AC bill.

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