We are prepping a Pullman roof and a/c ducts for some spot repair, and painting. We are cleaning off old, peeling caulk and hardened "tar" seam sealer, and power-wire brushing the roof and a/c duct panels. Most of the steel is rust-free, with a few spots of surface rust, which the power brushing has taken care of.

The seams will be sealed with urethane autobody seam sealer before priming and painting.

My big concern is having the primer and paint pop off the old galvanized steel panels, as seen in so many picture of these Pullmans. I'm sure they simply slathered on "car cement" back in the day. Ideally, we'd grit blast and tooth the surface, but we do not have that option on this project.

Some on-line research has turned up "OSPHO" as an etching treatment before painting (phosphoric acid). Has anyone had any experience with this stuff?

The thought now is to use an industrial alkyd high-solids primer, followed by urethane enamel topcoats.

Looking for some suggestions and ideas, based on experience.

Thanks,

Howard P.

I've used OSPHO as a rust converter, but not to give galvanized some teeth. It's phosphoric acid based, it may well work for you - please let us all know how it goes.

my instinct is that anything used to 'etch' galvanized would simply end up removing most of it. Zinc is used because it's more reactive then the steel (among other things) and strong treatment will cause it to react and degrade.

When I was a kid, we used a product called Galvaprep to etch galvanized steel gutters before we installed them on our house. The gutters were still there when we sold the house forty years later.

http://www.chemical-supermarket.com/galvaprep-5-zinc-phosphate-coating-p507.html

Just a thought. If you are close to the countryside, you might want to talk with barn painters.They do galvanized steel buildings all the time and most have good success. They may all have their own techniques or concoctions.

Hi Howard:
The Valley has been using: Pittsburgh "Interior/Exterior Galvanized Primer" for about 20 years. We are quite pleased as we have had no peeling, flaking or such. You can use it on non-galvanized steel as well, as in adjacent to your galvanized or on areas of the galvanized that have rusted. It is a bit pricy, but worth it.
J.David
PS: We are still top coating roofs with water base "Kool Patch" just like when I got here in 1986! JDC

Look at a variety of primers from reputable manufacturers and see which instructions seem most reasonable and the quality of the paint. A couple things on that.

First, any paint sales channel aimed at consumers is Right Out because all of it presumes two fatal things: #1 house paint, and thus very low VOC standards and more to the point, not designed to paint galvanized; and #2 that you are painting "new work" and thus your galvanized will be <1 year aged.

Second, your galvanized needs to be aged a year in the weather to develop a patina which is paintable. Painting freshly exposed galvanized is a much harder problem best "put off" a year. It's not like it's going to rust out in a year.

Of course it goes without saying that by painting galvanized, you are arresting its very function of oxidizing sacrificially to save the steel. Your paint coating must be near perfect to do this. Because any flaws in coverage or coating injuries will result in the zinc resuming its heroic task, working under your paint as it goes... *pop goes the paint*.

Also, you know they make "cold galvanizing compound" aka zinc paint. It is 90% zinc with just enough resin to hold that together. It is very heavy and needs lots of agitation, so buy it in quarts to not break your paint shaker. (having to hand stir a gallon is NOT fun.)

I used to work for commercial painting contractor. There are paints that will "stick" to galvanized metal. I've left a message for my Sherwin Williams rep asking for a product suggestion.

As noted above, the products are not things typically sold over the counter. They can be considered hazardous, depending on your area. Paint companies are somewhat reluctant to sell these products to just anyone off the street.

You will spend $75 to $100 a gallon on really good paint. Keep in mind that the stuff you buy at, say Home Depot for $40 a gallon is sold to commercial guys for less than $20.

Consider things like a water tower. You don't want to paint that every couple years, so you use really special paint at about $400 a gallon. It will last, but most people have a heart attack over the cost.

From Sherwin Williams --

The key to the successful painting of galvanized steel is surface preparation. It is most difficult to coat galvanizing 1 day to 6 months after galvanization. Weathering standard galvanizing for a minimum of 6 months is required to achieve a sound substrate for painting.

Paint adhesion to this final layer is generally very good as long as the
surface is clean. Always apply a test patch to evaluate adhesion. Test patch instructions are on the opposite page.

Surface Preparation
Galvanized steel needs to weather a minimum of 6 months prior to coating. Surface must be clean, dry and in sound condition. Remove grease, oil, dirt, soil and other contaminants using solvents, emulsions, cleaning compounds or steam cleaning per SSPC-SP1.

Do not use vinegar or other mild acids prior to painting, as these
are more aggressive and may damage the zinc surface.

After prep the Pro-Cryl Primer followed with DTM is a great system.

Keep in mind that the data sheets for this material clearly state this is FOR PROFESSIONAL USE ONLY. This is not something you typically pick up at your local store. I wouldn't let my guys use it without a respirator (not a paper dust mask!!). To comply with all the OSHA regs, you also have to be professionally fitted for a respirator and you have to be medically cleared also.

Can you brush this on? Yes, but it's much better to spray.

I'm not a huge fan of DTM (direct to metal); mainly because it's kinda thin. It takes multiple coats to get good coverage,especially if you want the finish to look good.

OSFLO is a good product. From a professional standpoint, we don't (officially) use it. We are concerned with a warranty from the manufacturer. You want to buy all the products from the same company. You can not buy this in certain states (PA for one). Read the data sheets on it. It's tricky to use.