Paint Galvanized Steel Reliably

A. Hi Dan. I seriously doubt that cold galvanizing compounds have some silver bullet approach to adhesion; rather I think they are just primers with a high concentration of zinc compounds in them. Thus I see no particular benefit to applying a cold galvanizing compound under your primer ... but I can't say whether a DTM paint from Sherwin Williams or a different primer made for galvanized and aluminum surfaces is better.

The overall question of adhesion of paint onto galvanized surfaces is rather complicated but is addressed in our FAQ "What is Galvanizing? -- and what if the paint won't stick?"

Luck & Regards,

A. Hi again. You may be right, but in that case I don't know what liquid zinc is, and can't say whether it sticks to the various types of factory galvanizing better or not. Sorry.

The SDS is at https://www.zrcworldwide.com/images/pdfs/msds/SDS_GHS_US_ZRC_Liquid_0220.pdf.

Luck & Regards,

A. Hi again. This is a public forum not a consulting service so "nobody seems to know" may no longer be true tomorrow, or next week, or next month, depending on who stumbles in & when :-)

I'm confident that Krud Kutter has competitors, but if you use vinegar I'd dilute between 1:1 and 2:1, probably no more. Whatever is going to happen will probably not take longer than a half-minute or minute ... then rinse.

You named 2 causes of poor adhesion but there is a third, perhaps even more important: the fatty acids in alkyd paint react with the zinc to form soaps, causing the paint to slide right off the galvanizing. Acrylic, latex, or other water-based primers are a much better idea than oil-based paints.

Luck & Regards,

A. So called "cold galvanizing" is zinc rich paint, and not really galvanizing at all. So called "factory galvanizing" is the real thing in that it can only be done in a factory, not a field applied coating, nor practical in an amateur fashion.

The problem with getting adhesion on a galvanized surface is a temporary one, and applies to fresh galvanizing - the problem goes away after perhaps 6 months, depending on the environment that the galv steel is in. During this time the zinc starts to oxidise, and it forms a complex of zinc oxide and zinc carbonates, and this stuff adheres really well to the zinc, and paint adheres really well to that oxidation product. So the lowest cost way (perhaps the laziest way?) to get paint adhesion to galvanizing is to leave 6 months between putting outdoors and painting. That's not always convenient. In factory applied powder coatings for example, that's often done the same or next day to the galvanizing. So something else is required. Sometimes called etching, other times a "conversion coating". Systems vary, butt the simplest of a phosphoric acid mixture with a little copper sulphate (often called "T Wash". The acid etches the surface, the soluphgate turns it black so you know what's treated and what missed. Let that dry and then paint. Various factory systems also exist to get adhesion, most involve an acid etch where tiny microcraters are created in the surface of the shiny zinc metal (the galvanizing). Fresh galvanizing can be like glass, and adhesion to glass isn't great as there's no anchors for the paint - no "key" for it to grip to, just a shiny smooth surface. Sandpaper prior to painting isn't just for smoothing, its to score the surface to get grip, adhesion.

There is a paint system developed in Belgium and sold round Europe developed especially for application to fresh new galvanizing. Various marketing names including "Galvacote", this is a silane paint system.

So using ordinary paint on fresh galv without some pretreatment is a recipe for paint loss. Using "cold galv" won't fix the problem, but might help a little. Best do it right!

A. Galvanized material can be easily painted successfully. Surface preparation is the key. You need to ask the paint manufacturer if your paint is suitable for a top coat for galvanized material and what the proper surface preparation should be. The paint will enhance the galvanized coating. It will act as a barrier protection for the galvanized material and subsequently the galvanized material will allow the paint to last longer because there is no underlying corrosion. Painting over galvanizing is often referred to as a "duplex coating." It is routinely done for street lights, DOT applications, and various architectural applications.

A. Yes, Rick, it is done routinely, but among other things galvanized finishes that are not going to be painted are chromated, whereas galvanized finishes that are to be painted are not. Painting a chromated galvanized finish is not done as easily -- but any galvanized finish can be painted. The American Galvanizers Association has several free on-line brochures.

Although URLs break or change over time, as of April 2015 the following URL leads to an excellent 12-page booklet called "Duplex Systems"
https://www.galvanizeit.org/education-and-resources/publications/duplex-systems-painting-over-hot-dip-galvanized-steel-2012

A. This is a tough question to answer without full context, Purnendu . . . because an individual consumer may be perfectly satisfied with a do-it-yourself paint job, but might find the same quality on an OEM part to be unsatisfactory.

When a consumer is trying to paint existing galvanized steel it may be hard to get good adhesion, but scrubbing with [on eBay or Amazon] and a tampico brush, followed by a self-etching primer made for galvanized steel may be good enough.

But when an OEM wants to paint galvanized steel, it starts with specifying the galvanized steel. Ideally it will be galvanneal, as used on auto bodies. If not, at least the final chromate treatment should be omitted. Then a conventional phosphatizing treatment at the factory. ASTM D6386 specifies the "Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting".

The American Galvanizing Association calls galvanizing followed by painting a "duplex coating" as Mike has advised, and they have several booklets with full details at www.galvanizeit.org. Good luck!

A. Hi Eric. The best pre-treatment is zinc phosphatization. Good luck.

Regards,

A. Hi Cynthia. We appended your question to a thread on the subject. I suggest scrubbing with TSP, followed by a "self-etching" latex paint made for galvanized surfaces, followed by your choice of type and color of paint.

Regards,

A. Hi Brett. Sorry I wasn't able to respond in timely fashion. I think your plan is/was ideal as long as you are sure that the primer is designed for this. You want a latex (water-base) primer because zinc reacts with alkyd chemicals, and "saponifies" (forms a soap layer) between the metal and the paint. I think the cold galvanizing sprays may be appropriate for patching small rust spots, but I doubt that they are necessary, or even as good, as a proper prime coat and top coat. Good luck.

Regards,