Painting Galvanized Sheet Metal

Duane WI

Member
I am going to build a new truck box for my 1952 Chevy. I can get 16 gauge galvanized sheet metal for about $85 for a 4x8 foot sheet. Non galvanized is about $10 per sheet cheaper. I need about 2 1/2 sheets to build the box. I am thinking to get the galvanized metal. There is not a lot of welding to put it together. Mainly spot welds in the corners. I can grind off the galvanizing in those areas before welding. I am thinking the painted areas would be scratched up good with sand paper, degreased and wiped down with phosphoric acid metal prep. I will be using epoxy primer with urethane top coat. Will the paint hold up well on the galvanized metal? I really like the idea of having all of the hidden rust prone areas protected by the galvanizing. Thanks for the help.
 
The degreasing is enough. You might use a cleaning solvent such as SSPC SP-1 Cleaning Solvent to remove any oils they may have put on it at the factory. I wouldn't use phosphoric acid. Galvanized is just a coating, it doesn't go through the metal and the phosphoric acid will remove a little.
 
Seems like Shermin Williams had at one time a galvanized prep, might have been copper sulfate? Or something. I think they could give you excellent professional advice.
 
You need a primer that is rated for galvanized steel.
I am not an expert but here is my opinion.
Ordinary primer (alkyd/oil based) will peel off in two years.
You stated you want to use epoxy primer.
I looked at technical data sheets (TDS)for several two part epoxy
primers. I didn't find one that mentioned galvanized steel substrate.
No news is bad news in this case. Looking at the TDS for urethane
primers, most mention galvanized steel as suitable substrate.
Urethane primers accept most high build block sanding primers and top coats
(as do epoxy primers also). Timing is critical you may have to scuff up
if too much time elapses between primer and top coat.
What are you going to do about cut edges, they will have no zinc coating.
maybe a spray primer with zinc such as ZincBrite.
 
I just checked, PPG Shopline Epoxy Primer specifically lists galvanized metal. I normally use Onmi Epoxy Primer which I think is the same stuff just different packaging. The Omni primer doesn't specifically say galvanized. I will have to call my paint guy and see what he recommends.
 
The two epoxies are the same. Shopline means that the dealer sells no paint but PPG paints.
 
Sure, just make sure you get the sheets clean and you won't have a problem. I painted a small building made with corrugated tin 20 years ago and the paint is still intact. I cheeped out used alkyd paint and the topcoat is badly faded but it's all still there.
 

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