Proper prep and painting for bare steel

[JackOfDiamonds]

I build a lot of bike frames from chromoly steel. What's the best way to prep and paint bare steel, anyway?

When I care about the results, I drop them off for powder coating. But I only do that rarely because most frames are prototypes or beaters and powder coating would double the cost.

I'm pretty happy using the "appliance epoxy" spray paint. I've been told it's not UV stable, but it's easy to spray in a couple coats or even one coat, and dries very hard.

Where I'm lost is the bare metal prep. The frame tubes have mill scale that's very durable, but I don't know if I need to remove it before priming.

Then there's at least some sections that are sanded bare metal too. The appliance epoxy says that primer is not needed. Should I believe that? If not, what primer to use? After I clean the flux off with hot water, the bare metal gets surface rust instantly. It seems wrong to paint directly over that, without some sort of converting primer.

 

[MP&C]

For painting I’d start with a good automotive epoxy primer like SPI. The mill scale needs to go, media blast with a garnet or coal slag abrasive to prep for the epoxy adhesion

 

[JackOfDiamonds]

Media blasting is not an option. I think mill scale was the wrong word. The tubes have a smooth durable grey oxide coating. I think it's called a pickled finish.

 

[Shiftless]

I paint a lot of bench vises.
After stripping off old paint and rinsing with water, surface rust forms quite quickly. I dry parts off with an electric heat gun and coat with thinned down Rustoleum rusty metal primer. After that cures, you can paint right away or wait as long as you want. I have top coated with alkyd paints or acrylic paints. They both stick quite well.
But I then keep them inside so I can’t testify about weathering but that’s dependent on the top coat.

 

[MP&C]

Media blasting is not an option. I think mill scale was the wrong word. The tubes have a smooth durable grey oxide coating. I think it's called a pickled finish.

That is indeed mill scale. If it is hot rolled and “pickled”, that refers to an acid bath that is used to remove the grey scale where you’d see bright metal. If you can get cold rolled tubing, it doesn’t have the mill scale, and is probably a better choice for what you are doing.

 

[Deadsquiggles]

When I deal with hot rolled, I get the mill scale off, usually with a scotchbrite disk or flap disk on a angle grinder or die grinder depending on the size of the piece, then wipe it down with acetone, then spray a coat of primer, let it dry, then top coat. Haven’t had an issue with spray coming off for any reason other than use.

 

[JackOfDiamonds]

I've seen tubes that are "bright". These are typically special tubes for bicycles. But all of the aircraft tubing that I buy is universally the brown/grey color. It's tenacious and could be mistaken for a thin hard coat of paint, but it's oxide. It wouldn't bother me at all to paint over it. It seems counterproductive to remove it and I can't remove it anyway because you would have to blast it off.

Also I have experimented with pickling in warm citric acid. This works very well on the flux and leaves the whole frame with an even black coat of "****" which can be washed off to reveal bare metal but you get that instant flash of surface rust that way anyway.

I will look into the "rusty metal" primer as a first coat.

 

[Shiftless]

The Rustoleum rusty metal primer is really sticky which is good. I buy it in the half pint cans even though it’s more expensive per ounce. It has a limited shelf life in partially full cans so unless you’re planning on a LOT of bike frames per year, buy the smaller cans.
It skins over. I suppose you could carefully remove the skin and use the paint underneath if you had to. I have done that successfully.

I brush it on but I suppose you could thin it properly and spray it on with an air brush.


.

 

[HMCFab9]

Clean any oil etc. residue off.
Sandblast to get the mill scale off.
Spray with a automotive 2 part epoxy primer.
Paint with whatever you like.
Automotive paint would be the best option.

 

[Graham08]

That is indeed mill scale. If it is hot rolled and “pickled”, that refers to an acid bath that is used to remove the grey scale where you’d see bright metal. If you can get cold rolled tubing, it doesn’t have the mill scale, and is probably a better choice for what you are doing.

There isn't an option to get cold rolled 4130 tube. It's seamless and has some scale from the drawing process. It's nothing like the scale on hot rolled plate or tube...It's actually very smooth and shiny from most mills.

I've seen tubes that are "bright". These are typically special tubes for bicycles. But all of the aircraft tubing that I buy is universally the brown/grey color. It's tenacious and could be mistaken for a thin hard coat of paint, but it's oxide. It wouldn't bother me at all to paint over it. It seems counterproductive to remove it and I can't remove it anyway because you would have to blast it off.

Also I have experimented with pickling in warm citric acid. This works very well on the flux and leaves the whole frame with an even black coat of "****" which can be washed off to reveal bare metal but you get that instant flash of surface rust that way anyway.

I will look into the "rusty metal" primer as a first coat.

The bright tubes are most likely centerless ground to remove the scale and slightly reduce weight.

It sounds like you're looking for a low cost option for preventing rust. If you're not having an issue with the appliance epoxy sticking, I would continue to use it without removing the scale from the tubes.

If adhesion is a problem, the best way to fix it would be to media blast and prime with a proper epoxy. Or...I have had good success with a product called Steel-It that comes in spray cans. It's expensive compared to Rustoleum, but cheap compared to epoxy primer and urethane paint. Steel-It requires a blasted or rough surface to adhere properly, but if you do the prep it is very durable. It is 316 stainless flake in an epoxy binder, and only comes in silver or black.

 

[JackOfDiamonds]

Low cost option...yes
Preventing rust...sort of but not really. I live in a desert and the other day my kids asked me "what is rust?"

I never had a problem with paint adhesion or rust.

I went ahead and gave it the hot water bath. It flashed immediately to light rust. Then when it was dry I sprayed it with Rust-oleum primer. The can says I need to wait until 48 hours before recoating. I'm impatient so now I'm sort of thinking I should have sprayed the appliance epoxy over the rust and saved 2 days of waiting.

 

[metlmunchr]

Acid pickling of 4130 tubing isn't a good idea. The chemical reaction releases free hydrogen which can cause hydrogen embrittlement in certain steel alloys, and 4130 is one of the alloys that's highly susceptible to embrittlement.

Because some reactions are time dependent, pickling in a weak acid could be more harmful than pickling in a strong acid for a shorter time. Note I said "could", as that isn't something I know as a fact but rather a possibility that could only be answered accurately by a metallurgist.

Bottom line though, is if an item needs the strength of 4130, then any sort of pickling should be avoided. If scale has to be removed, it should only be done by mechanical means.

 

[csp]

Paint stripping discs on an angle grinder work great for removing any kind of mill scale. They're 100x better than a DA sander or flap disc.

 

[OldNeons]

One option to consider is the Hammertone spray cans. No primer required and forms a nice durable finish. Almost has its own clear built in. Experience in application will allow more or less texture in your finish. Adequate surface prep to insure adhesion is essential to any coating that you want to last.....

 

[Mr.N]

One option to consider is the Hammertone spray cans. No primer required and forms a nice durable finish. .

Durable is in the eye of the beholder...

I use to paint for an Aerospace company.
Spray is WAY better than from a can and I never use a rattle can. Follow the directions if you are going to spray, you need a lot more than just the paint. That said my parts rarely get sprayed... I heat the can paint can to 100+*
Also, I like the 2 part epoxy as my primer. Wipe down with a clean just before, again read the directions the actually tell you what to use.

I have been using Citric Acid lately to remove the mil scale/ pickle. Wish I found it ealier... As said, you need to move quickly when removing out of the bath.
Really, the quality of steel very a lot, so I like to shop around.