Repainting 100-year-old galvanized steel

[scott1971]

Hello fellow engineers!

I live in a 100-year-old building with painted galvanized steel siding. The paint is severely damaged and is flaking off (please see pics). There are areas of surface rust, but nothing I would consider serious. I do not know if this is the original paint or if/when the siding was last painted.

I sit on the condo board and there's been much discussion about having the siding scraped, possibly power-washed, and re-painted. I have concerns that the scraping and power-washing may remove the zinc patina that has formed and is currently protecting the steel from aggressive corrosion. Are my concerns valid, or will traditional surface preparation be okay? My research has turned up some conflicting opinions on surface-prep and which primers and topcoats to use. Obviously, the flaking paint must be removed, but how does one do this without damaging the galvanic coating? I don't want to spend over $43K and later find out we made the problem worse. Any recommendations on how to best approach the repainting are appreciated.

Scott
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[unclesyd]

First off is that the galvanize is old so it could contain an appreciable amounts of lead, so be careful how you approach the problem.

A little more information needed.

How much surface area are you talking about?

What is the environment around the building?

The paint looks like a well oxidized epoxy or one of the original water based paints, like Chem-Tone.

 

[scott1971]

The area to be painted is quite large. It is an 8-story building and the siding is under and between windows on each floor on three sides of the building. I estimate about 4000 sq.ft. Click here for a pic of the building. Everything that's gray needs to be painted. You can see that it's not just siding, but also detailed trim on the upper floors.

I don't know if lead was used during the galvanization, but the paint itself is likely lead based. The contractor we select would need to take whatever precautions are needed. I don't know anything about the paint, but if it was water-based, I would think it would have washed away by now.

The building is in northern New Jersey, a suburb of NYC (assuming this is what you meant by "environment"). Just your everyday neighborhood.

Will using paint scrapers be okay? How about wire brushes? Or maybe scraping should be avoided and we should only power wash? Since there are already areas of rust where the zinc has presumably been consumed, maybe scraping won't compromise the galvanization any worse than it already has been. I really don't know. Just trying to make sure we make a sound investment. I'm worried that a typical residential paint contractor might make the problem worse if they're not experienced with painting galvanized steel.

Scott
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[unclesyd]

I just talked to a old colleague who worked for a local industrial painting company and his advice was to check around your area as there should be someone who specializes in such work and has to equipment to facilitate the work. He also stated that a job such as this can turn sour in a hurry, like the paint being lead based and corrosion to the backside of the siding, bad attachment points, etc.
I'll try to contact friend who works for the Port Authority of New York and see if he knows anyone in the area that does this type work. He travels all over the NE.
I would check with the local zoning board to see if the know of any company component enough to give advice and do the job.

 

[unclesyd]

Just thought of something depending on the age you may have tin plate or terne (lead and tin alloy) plate siding. It really doesn't make that much difference as both are treated essentially the same when restoring. It is just another bit of information that needs to be known.

Hopefully it will not be a lead based paint, but an oil based alkyd like the Calbar product.

Here is one product that is used for such work. You might want to contact them with your conditions. Also they maybe able to recommend one or two contractors that can do the work if you decide to restore it.

http://www.calbarinc.com/product_description9.html

I don't know about galvanized but I think you can still buy terne plate siding.

 

[scott1971]

I have it on good authority that the siding is galvanized steel and the base metal has the spangle patterning typical of galvanization. We definitely don't want to replace the siding itself.

I've read that alkyd paints are not suitable for direct application to galvanized steel (i.e., must use non-alkyd primer).

Scott
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[unclesyd]

Being galvanized certainly removes some constraints on any type of restoration.

If you let the galvanize age for a year or so you can paint suing some alkyd paints. As you state you can paint earlier if you use a primer. In my opinion any time you paint paint any type of metal you should use a primer.

As I posted I would contact Calbar and come up to speed on their siding paint.

 

[Compositepro]

I think that the only way you will get a meaningful answer is to ask for bids, with warranties, from painting contractors.

 

[hokie66]

The galvanized surface is plenty old now. But you still need a primer suitable for galvanizing, and additional treatment at the rusted areas.

Removing the old paint is the main problem. It will probably come off with heat, but the lead issue has to be investigated and solved.

Alkyd topcoats are not the longest wearing for external surfaces. Acrylics have must better weathering properties. An acrylic should give 10 years service before recoating.

Any painter capable of a job that size will likely know how to approach the problems. A house painter is not going to tackle it. But you still need to investigate other jobs they have done.