Powder coating sheet metal

[mmkid]

Do guys have sheet metal sandblasted and powder coated?
I am by no means a painter. And I don?t wsnna wire wheel all
this stuff really. I?m not concerned about a few imperfections in
the metal that would look better if they were bondoed and
sanded. Or is sand blasting to extreme and could damage it?
Thanks. Just had a couple sets of 9 bolt rims sand blasted
and powder coated and having this done just seems like the
way to go.

 

[yakob]

(quoted from post at 08:09:42 04/03/19) Do guys have sheet metal sandblasted and powder coated?
I am by no means a painter. And I don?t wsnna wire wheel all
this stuff really. I?m not concerned about a few imperfections in
the metal that would look better if they were bondoed and
sanded. Or is sand blasting to extreme and could damage it?
Thanks. Just had a couple sets of 9 bolt rims sand blasted
and powder coated and having this done just seems like the
way to go.

Sand blasting can damage sheet metal but that doesn't mean that it will. It has more to do with the technique and blasting material than anything.

You might want to see how the rims hold up over the next 5-10 years before you go all in on powder coating. I've read a lot of posts where guys coated wheels and were very happy like yourself but then once a scratch finally does happen, the coating just flakes back from there.

 

[North Maine]

(quoted from post at 09:09:42 04/03/19) Do guys have sheet metal sandblasted and powder coated?
I am by no means a painter. And I don?t wsnna wire wheel all
this stuff really. I?m not concerned about a few imperfections in
the metal that would look better if they were bondoed and
sanded. Or is sand blasting to extreme and could damage it?
Thanks. Just had a couple sets of 9 bolt rims sand blasted
and powder coated and having this done just seems like the
way to go.

Why not sand blast and then paint? That's what I'm doing to my 3020... I'm not a fan of wire wheeling, plus the mechanical adhesion that occurs with epoxy over a blasted surface can't be beat!

 

[showcrop]

I have a number of things that are powder coated. The thrower on my baler, some parts of some tractors that I have repainted, my 22,400 tandem dual trailer and some others. The problem with it is if it is out in the weather and gets scratched, the rust spreads much faster under it than it does under paint. If you were to prime it first with paint it would last much better, but then you lose the advantage of powder coat.

 

[504]

I keep looking, but I just keep seeing huge pieces of power coat falling off,with rust 1/8 thick
going with it. Give me paint. FYI the powder coat company's here sand blast everything before they
coat.

 

[Stephen Newell]

Sand blasting or using a wire wheel would only help adhesion. The only thing that is different about powder coating is instead of spraying a liquid primer on the metal the metal is charged with static electricity and the polyester powder is sprayed on. Then the metal is put in an oven so the powder melted together and to the metal.

 

[North Maine]

(quoted from post at 22:37:45 04/03/19) Sand blasting or using a wire wheel would only help adhesion. The only thing that is different about powder coating is instead of spraying a liquid primer on the metal the metal is charged with static electricity and the polyester powder is sprayed on. Then the metal is put in an oven so the powder melted together and to the metal.

Yes that is the process, however that “only thing that is different” is a significant difference. Powder coating frequently suffers undercutting at a scratch or chip that leads to large scale coating failure. While this can also happen on a traditionally painted panel it is far less likely on a surface that has been blasted and epoxy primed before topcoating.

 

[yakob]

(quoted from post at 08:55:23 04/04/19)

(quoted from post at 22:37:45 04/03/19) Sand blasting or using a wire wheel would only help adhesion. The only thing that is different about powder coating is instead of spraying a liquid primer on the metal the metal is charged with static electricity and the polyester powder is sprayed on. Then the metal is put in an oven so the powder melted together and to the metal.

Yes that is the process, however that “only thing that is different” is a significant difference. Powder coating frequently suffers undercutting at a scratch or chip that leads to large scale coating failure. While this can also happen on a traditionally painted panel it is far less likely on a surface that has been blasted and epoxy primed before topcoating.

Yeah, that "only thing that is different" followed by a list of differences made me giggle a bit.

I've heard of some powder coaters putting epoxy primer on before doing the powder. I don't know how the long term durability of this is...it would make sense that now there is a "protective" layer to keep rust from spreading, but at the same time is it OK for epoxy primer to be baked on that hot???

 

[North Maine]

most epoxies hit their "glass transition temperature" where they chemically start to breakdown around 140F though I know of some marine industry tank linings that are rated for 180F. Never anything as high as powder coat baking temps thought.

 

[yakob]

(quoted from post at 14:53:16 04/04/19)
most epoxies hit their "glass transition temperature" where they chemically start to breakdown around 140F though I know of some marine industry tank linings that are rated for 180F. Never anything as high as powder coat baking temps thought.

A quick google shows me one that is rated for 400F:

http://www.ppgaerospace.com/Products/Coatings-Removers-Cleaners/Commercial-Civil-Aviation/Specialty-Products/DeSoto%C2%AE-519X303-High-Temperature-Epoxy-Primer.aspx

Looks expensive

 

[paintron]

My current employer paints a lot of sheet metal, millions of pounds. Paint won't properly stick to most of it, with a few exceptions, without a pretreatment. Zinc phosphate is by far the best, many chrome based treatments also work or you could use a wash primer which is a treatment and primer in one.

Prime it next, usually a epoxy or urethane are good choices through their are others, then powder coat.

 

[yakob]

(quoted from post at 12:44:26 04/05/19) My current employer paints a lot of sheet metal, millions of pounds. Paint won't properly stick to most of it, with a few exceptions, without a pretreatment. Zinc phosphate is by far the best, many chrome based treatments also work or you could use a wash primer which is a treatment and primer in one.

Prime it next, usually a epoxy or urethane are good choices through their are others, then powder coat.

I'm going to have to bite on this one:

What on Earth is going on that they can't get paint/primer to stick without pretreatment?

In auto refinishing the only time a treatment is really mentioned is when we are trying to dissolve away rust that is in a stubborn place or where sandblasting isn't going to work, etc...

Epoxy gives you a mechanical bond. I'm assuming that in your industry blasting/sanding is not suitable for some reason?

 

[paintron]

When us OEM guys get together, you would not believe the humour when we discuss the various shows on TV where cars motorcyles etc. painted are repainted. Mechanical adhesion is only on type of adhesion and not remotely as effective as chemical bonding.

Paint on it's own is porous and will not stop corrosion for any length of time. Fortunately I have had access to salt spray chambers in most of my previous and current position and have seen first hand how various treatments vs no treatment effect the finished product. .

 

[yakob]

(quoted from post at 11:58:03 04/12/19) When us OEM guys get together, you would not believe the humour when we discuss the various shows on TV where cars motorcyles etc. painted are repainted. Mechanical adhesion is only on type of adhesion and not remotely as effective as chemical bonding.

Paint on it's own is porous and will not stop corrosion for any length of time. Fortunately I have had access to salt spray chambers in most of my previous and current position and have seen first hand how various treatments vs no treatment effect the finished product. .

So you are saying that epoxy has a chemical bond with pretreatments that have been applied and cured before moving to the epoxy? Sure, the treatment reacts chemically with the metal, but the next step is not another chemical bond.

Of course certain treatments will affect the finished product in certain ways, but it is not always relevant or necessary. If I'm painting a medicine cabinet then I can probably get by with Rustoleum for 100 years on it because it stays dry and out of UV light. Some of your "industrial" applications may need every last bit of help they can get for corrosion resistances, but a tractor (esp. a restoration)? Come on. Even the synthetic enamels and primers the equipment dealers sell aren't going to show you corrosion issues. The salt spray test simulates just that -- salt spray.

I'm not against the treatments totally. They are useful in a lot of cases. Mostly I'm interested them for the under-body and frame of a car. A blanket recommendation for them is adding extra cost, hassle, and room for error, especially for a typical DIY'er.

 

[David G]

I personally do not believe powder coating adheres as good as primer/paint, have seen two much peel off.

 

[showcrop]

(quoted from post at 18:33:35 04/12/19) I personally do not believe powder coating adheres as good as primer/paint, have seen two much peel off.

Yes David, you and pretty much everybody else, LOL. The only reason for the popular use of powder coat is that it gets manufacturers out from under the very expensive VOC rules.

 

[paintron]

I'm not against the treatments totally. They are useful in a lot of cases. Mostly I'm interested them for the under-body and frame of a car. A blanket recommendation for them is adding extra cost, hassle, and room for error, especially for a typical DIY'er.[/quote]

Like many of you I like doing DIY projects, though my tractor is a garden tractor. Along with my home property I own a large lot in a small town about an hour’s drive away which needs to be maintained and a second 17 acre lot with lots of bush rock (Canadian Shield) and cabin three hours away. One project was making a loader for my tractor, which obviously required painting. Future projects include a dump trailer and a towable backhoe. I’m an expert at industrial painting which got me putting procedures together that can be easily be done by other do it yourselfers.

I familiar with the all the products, basic equipment and supplies needed for painting and where to purchase them. Stuff that another DIY would not have known exists.

Incidentally, I used to live on a farm.

 

[yakob]

(quoted from post at 13:00:29 04/15/19) I'm not against the treatments totally. They are useful in a lot of cases. Mostly I'm interested them for the under-body and frame of a car. A blanket recommendation for them is adding extra cost, hassle, and room for error, especially for a typical DIY'er.

Like many of you I like doing DIY projects, though my tractor is a garden tractor. Along with my home property I own a large lot in a small town about an hour s drive away which needs to be maintained and a second 17 acre lot with lots of bush rock (Canadian Shield) and cabin three hours away. One project was making a loader for my tractor, which obviously required painting. Future projects include a dump trailer and a towable backhoe. I m an expert at industrial painting which got me putting procedures together that can be easily be done by other do it yourselfers.

I familiar with the all the products, basic equipment and supplies needed for painting and where to purchase them. Stuff that another DIY would not have known exists.

Incidentally, I used to live on a farm.[/quote]

I didn't see that you attempted to quote part of my last message.

I can't tell if that is some form of rebuttal or just a brag on your credentials. Trust me -- I'm not trying to lock horns and bicker unnecessarily. I want people to have good information.

You keep mentioning pretreatment as being needed, creating chemical bonds, fixing issues where adhesion was a problem, etc., then pivoting when that view is challenged.

My point of view is:
1. Primer will adhere without treatment and treatment CAN cause issues if not done properly. Sure you outline what you call a sound procedure, but regardless of how sound that procedure is, it is an extra set of steps that introduce room for error and disappointment.

2. Epoxy primer will STILL be a mechanical bond to the treatment is. Any time a urethane is applied and then sanded, the next coat is a mechanical bond. Body filler is a mechanical bond. At some point the "weakest link" argument is relevant.

3. I don't have any reason to believe that paint/epoxy is "porous" in any measure relevant to the application in this forum. Please elaborate "paint on its own will not stop corrosion for any length of time." We are not immersing something with rustoleum in salt water and hoping it is still there in 100 years. The use case is relevant to the procedure.

 

[paintron]

Sorry, not trying to lock horns either, just surprised that a very simple process, clean, rinse, and apply is seen as onerous. The beginning of the thread was an adhesion
issue and lack of treatment is the most likely cause, I even agreed with your fix. I just don't want anybody else to repeat this if they are restoring a old tractor which
deserves to be persevered for future generations.

Keep in mind most job shops use recycled rinse water as a DIY project in your driveway this process is very hard to screw up.

 

[paintron]

If looks as if your powder coated did not put any treatment on your sheet metal prior to painting.

I have a background in chemical engineering, and a secondary education in coating's. Along with over forty years in the application of coating's. If your interested I can help you walk through the issue and have it fixed.

 

[yakob]

(quoted from post at 11:25:27 04/17/19) Sorry, not trying to lock horns either, just surprised that a very simple process, clean, rinse, and apply is seen as onerous. The beginning of the thread was an adhesion
issue and lack of treatment is the most likely cause, I even agreed with your fix. I just don't want anybody else to repeat this if they are restoring a old tractor which
deserves to be persevered for future generations.

Keep in mind most job shops use recycled rinse water as a DIY project in your driveway this process is very hard to screw up.

I think you are blurring together 2 threads. The OP in this one did not claim an adhesion issue. If I recall, the first adhesion issue was actually mentioned by you citing issues your employer had "getting paint to stick to metal."

I'm not claiming that your process is inherently difficult or flawed in any way. I'm saying that it is more steps that are often unnecessary and more steps = more error. "Driving to the grocery store" is a simple task but I am less likely to hit a deer if I stay home with the car in the garage. It's simple statistics.

Powder coat is a mechanical bond. Epoxy primer is a mechanical bond whether it be on sanded metal or treated metal. Treatments that leave behind a "conversion coating" NEED to be prepped before applying epoxy to them and absolutely need to be neutralized to not cause naughty issues with certain epoxies. Properly sanded metal is a better mechanical adhesion than a hard, smooth coating on the sanded metal. Sure, the conversion coating is a strong chemical bond.

Pre-treatment is NOT "THE" solution to adhesion issues. Period.

 

[paintron]

i think your right about mixing up the two threads, sorry. I your opinion is treatment does not effect adhesion no problem, my opinion is based on science and experience.
The readers of this site can decide for themselves whom they want to believe.

 

[yakob]

(quoted from post at 09:48:12 04/18/19) i think your right about mixing up the two threads, sorry. I your opinion is treatment does not effect adhesion no problem, my opinion is based on science and experience.
The readers of this site can decide for themselves whom they want to believe.

Can you address the science behind why epoxy primer would mechanically bond to a treatment or treated metal vs properly sanded metal?

 

[yakob]

(quoted from post at 09:48:12 04/18/19) i think your right about mixing up the two threads, sorry. I your opinion is treatment does not effect adhesion no problem, my opinion is based on science and experience.
The readers of this site can decide for themselves whom they want to believe.

Not made up opinions by any stretch. I don't find it valuable to throw around credentials on forums because, at the end of the day, I could pretty easily be making them up.

Link to another thread involving adhesion issues after a rust treatment:

http://www.spiuserforum.com/index.php?threads/spi-epoxy-adhesion-problems.4944/

There absolutely are cases where various treatments/conversion coatings cause issues with primers and you seem to be in denial about this fact.

 

[yakob]

(quoted from post at 09:54:22 04/18/19)

(quoted from post at 09:48:12 04/18/19) i think your right about mixing up the two threads, sorry. I your opinion is treatment does not effect adhesion no problem, my opinion is based on science and experience.
The readers of this site can decide for themselves whom they want to believe.

Not made up opinions by any stretch. I don't find it valuable to throw around credentials on forums because, at the end of the day, I could pretty easily be making them up.

Link to another thread involving adhesion issues after a rust treatment:

http://www.spiuserforum.com/index.php?threads/spi-epoxy-adhesion-problems.4944/

There absolutely are cases where various treatments/conversion coatings cause issues with primers and you seem to be in denial about this fact.

Note: other adhesion issues come up in the discussion, but the fact is you can find numerous references back to DO NOT APPLY certain primers over these coatings.

 

[rdandersom]

Not intending to step on any toes but there are 2 issues not addressed so far. My experience was in a commercial paint shop mostly doing OEM parts. We used iron phosphate and iron phosphate/zinc pretreatment. I was leadhand for 6 years. I have no experience with home shop powder coating.I have 2 ideas to contribute. Firstly that most powder coat material is actually quite brittle especially when applied thick..For this reason it is important to keep the finished coat thin so that it will bend with the metal. We aimed for a finish thickness of 2.3 microns.Secondly the temperatures for curing are not air temperatures in the oven but the temperature of the surface of the metal. Our cure check on the line was to take a part off at the unloading station and rub it 10 times with an acetone dampened rag. A little bit of transfer is acceptable as the cure process actually continues for hours after cool down.

 

[paintron]

You're correct, temperature is measured as PMT, peak metal temperature. I've never worked with powder coating, one of the few holes in my experience. Suprised that you use acetone for measuring cure, MEK is normally used as it evaporates a little slower and keeps the cheese cloth wet.

 

[paintron]

Sorry I'm having issues moving through the thread, home computer, work computer, tablet. Some restrict my
movement.

Adhesion involves hydrogen bonding and Van Der Vaals forces. These are much stronger with some molecules that
others.

 

[paintron]

Your correct, however spray painting also use electrostatic application. See ransburg. The end polymer is the same in both system, polyester is polyester irrespective of how it is applied. Powder is a thicker not always a advantage. They also like to use thermoplastic systems along with thermoset, the liquid segment use thermoset almost exclusively.

 

[rdandersom]

It could have been a customer specification. We were ISO certified to do work for suppliers to the big 3 auto companies and my standard procedure manual specified acetone. I believe the work dockets for high end GM parts like Hummer spare tire carriers and corvette water pump pulleys specified that test. I know the auditor watched me do it during the annual ISO certification audit.

 

[Bonnan]

Great thread and would just like to thank you both for the most civil/thoughtful discussion between two technical positions. As a retired plant engineer I sat in on many a meeting between the two and was responsible for making real life decisions there from. Always interesting and challenging.
Jim B (still learning and doing projects)

 

[paintron]

Your right these threads tend to get personal sometimes. I like tractors and would like to see them preserved, and a such enjoy passing on my expertise to those interested in using it. Only a few months to go until I retire looking forward to checking out some tractor museum collections, orange in particular.