Coating for galvanized steel

[lucky-guesser]

This is perhaps not the correct category for this, but I don't exactly know how to classify this as there are several issues and I know it will be seen here...

We produce water storage tanks made from galvanized steel, which is common practice in our industry but it does rust/corrode/leak eventually. We are hoping to get more life out of these tanks by spraying on either some sort of sealant or corrosive barrier to the entire inside surface of the tanks. One of my coworkers did a little testing a couple years ago related to coatings, but nothing extensive and with a different goal in mind so there's not a whole lot I can take from it.

Question to you all is, is there a certain coating that you would recommend, preferably one that you have personal experience bonding to galv. We would potentially be using hundreds to a few thousand gallons a year.

Since galvanized is the bane of my existence, question 2 is what did you do for prep work? One coating company recommended shot peening for their coating but another company said that their coating requires a more abrasive blasting to rough up the surface more, so I think the prep work is also pretty coating specific. What do you use to quickly/efficiently clean the surface between blasting and coating?

 

[dvd]

galvanizeit.org/education-and-resources/publications/duplex-systems-painting-over-hot-dip-galvanized-steel

 

[EdStainless]

dvd is on the mark here.
There are coatings designed to go over galv.
They are nearly all (at least the good ones) two step with a primer that chemically bonds to the Zn and then a top coat.
Coating work is tricky.
Cleaning is 99% of the job, and environmental control (temp, humidity) is the other half.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[lucky-guesser]

galvanizeit.org/education-and-resources/publications/duplex-systems-painting-over-hot-dip-galvanized-steel

They gave some good insight with some things to look out for, I like the table with the different paint types, but I could still use some more specifics as far as actual implementation.

Also, a lot of the paints that I am seeing are made to be rolled on, which we are really hoping to spray as a time saving method. Do you think these could be sprayed with enough PSI behind them?

 

[EdStainless]

You will need to contact the coating mfg to find out about spraying.
(don't call them paint, they get touchy about this)
You also need to plan on a system of QA.
You need a way to test the cleaning, test the coverage of the first coat, and then test the coverage of the second coat.
If this is potable water you will need coatings approved for this service.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Most industrial coatings are meant to be applied with airless sprayer. If the coating manufacturer is feeling generous you may be allowed to apply it by brush or roller.

Tank interior coatings are typically a very high build epoxy formulation or polyurethane rubber.

Does this need to be NSF 61 for drinking water?

 

[Stick.]

what did you do for prep work?... What do you use to quickly/efficiently clean the surface between blasting and coating?

Whatever the coating manufacturer says to do and nothing else. That's the answer to basically any process questions when it comes to coatings. The reason for following their instructions exactly, with zero deviations is twofold. First, they know their coatings and how to use them, so you're more likely to get a good result if you follow their instructions. Secondly, if your customer comes back to you with a coating failure, and you in turn go to the coating manufacturer to complain that their coating failed, the first thing they'll ask about is your whole process of applying the coating (surface prep, application, etc). If you skipped a step, used a different solvent from what they specified, or any other deviation, they'll pretty much tell you that you screwed up and it's your fault. Any quality coating manufacturer will have documentation that lays out the whole process for all of their various coatings.

Once you have some options for different coatings, I've found it helpful to make a chart to compare all the different aspects of each option (things like what cleaning solvents it may require, what surface finish it requires, how it's applied, what's allowed for touch ups, etc).

 

[lucky-guesser]

Most industrial coatings are meant to be applied with airless sprayer. If the coating manufacturer is feeling generous you may be allowed to apply it by brush or roller.

Tank interior coatings are typically a very high build epoxy formulation or polyurethane rubber.

Does this need to be NSF 61 for drinking water?

I might just not be searching for the right thing. I keep finding gallons jugs of stuff that you can order online, having a harder time finding an industrial site.

From what I have read, epoxy doesn't have good UV resistance so since the tanks are open to the sun 24/7 that's a no go, but I will look into the polyurethane.

It doesn't have to meet any defined standards that I know of as its not "potable" water but livestock will be drinking from it so it does need to be non toxic. I found one coating that was perfect until the manufacturer said it was "very toxic".

 

[TugboatEng]

They all have parallel product lines so pick whichever has a local rep. A local rep will save you a lot of hassle dealing with availability and regulatory issues.

International
PPG
US Coatings
Jotun
Sherwin-Williams
Etc...

Some of these have consumer and automotive product lines. Don't get them confused. You want the commercial or industrial divisions.

If you're feeding livestock you'll still want to use NSF 61 coatings. You won't be able to sell the meat if it's been hydrated with toxic water.

Surface tolerant epoxies do exist. They develop a chalky appearance and fade color wise but this doesn't affect the performance of the coating.