Mill Scale Removal for paint

[Kaizen]

Working on a large project and having an issue with the mill scale. I normally couldn't care as its shop use stuff but this will be a fence outdoors and i'd rather not have to repaint it.
I cannot media blast it nor use a dip method as its too big. I also cannot move it outside and blast it. I've used flap disc, grinding wheel, grinder with sandpaper disc, eastwood drum removal thing, and just bought an adjustable grinder and silicon carbide cup to run at low speed. Most get down to shiny metal but then clog up after a few seconds. Others just seem to polish the surface. So is it necessary for paint or if i am putting 40 grit scratches, metal primer, and rustoleum will it be good enough? Really have to gouge it to get it shiny as shown.

 

[Jayman17]

I’m anxious to hear what others say because I have experienced the same thing.

Jay

 

[PCustoms]

I don't see mill scale.....

Square tube usually doesn't have scale. Flat stock and angle does. Paint prep discs eat it right up, but are $$.

https://www.amazon.com/dp/B00NOESL2I/?tag=atomicindus08-20

If you are clogging I suspect the tube is just dirty.

 

[Kaizen]

I’m anxious to hear what others say because I have experienced the same thing.

Jay

its strange. I prepped the pickets on this fence and 1 basic flap wheel did all 42 of them. Then on these 2 1/2 inch pieces its like armor. I saw something on Weld.com where they had a diamond encrusted cup but at 350.00 thats more then i care about

 

[Kaizen]

I don't see mill scale.....

Square tube usually doesn't have scale. Flat stock and angle does. Paint prep discs eat it right up, but are $$.

https://www.amazon.com/dp/B00NOESL2I/?tag=atomicindus08-20

If you are clogging I suspect the tube is just dirty.

I'm assuming they are clogging as they work for a few feet and then drastically decrease effectiveness. What do you mean dirty? oil and such? I wiped everything down with acetone before welding. If not mill scale is it just discolored?

 

[PCustoms]

I'm assuming they are clogging as they work for a few feet and then drastically decrease effectiveness. What do you mean dirty? oil and such? I wiped everything down with acetone before welding. If not mill scale is it just discolored?

What abrasive are you using?

Yes, by dirty I mean oil and crud. If you are wiping down that should remove it.

Post a closeup of the raw material. If your top pic is the raw tube that is ready for primer and paint, there is no scale there.

And take a look at the link. The woven prep pads are the ticket for mill scale and paint prep. Very little metal removal and they don't clog.

 

[BukitCase]

The link text calls it Freud Diablo, which is Freud's upper end stuff; but the pic shows "Avanti-Pro, which in my experience equates to Sears' old line "even at this low price", IOW GARBAGE - local Homeless Despot didn't have the Diablo versions of a couple different products, so I made the MISTAKE of bringing the Avanti stuff home - total waste of time, money, effort, etc...

Best luck I've had with mill scale on flat bar and angle is to start with a thin cutoff disk on 4-1/2" grinder, disk at about 45* angle and rapid movement across the piece til I get MOST of the scale removed - then a 40 grit flap disk to smooth out.

For larger projects (using tube) I found that getting the oil off went halfway easy by laying full 20' lengths of tube side by side on horses, spraying it down with Costco's de-greaser stuff then using a small floor buffer (mine's a "Gloss Boss" - I use the stiff brushes on the buffer and keep rotating all the tubes to get the other sides, then hose the whole thing down with water and let dry (summer sun helps :=)

Downside - don't wait too long before fabbing up and painting, or the NEXT problem is "how do I get the rust off"

The WINTER version of above: replace "summer sun" with a heat gun blowing thru the tubes; not ideal, but neither is "life"...

Best I can offer for an un-solveable problem... Steve

 

[Kaizen]

What abrasive are you using?

Yes, by dirty I mean oil and crud. If you are wiping down that should remove it.

Post a closeup of the raw material. If your top pic is the raw tube that is ready for primer and paint, there is no scale there.

And take a look at the link. The woven prep pads are the ticket for mill scale and paint prep. Very little metal removal and they don't clog.

I think you are correct that i did not wipe down these pieces. I wiped and ground where i was welding but not the rest. I'll wipe and give it another shot. Here is the virgin material and after grinding.

 

[Kaizen]

The link text calls it Freud Diablo, which is Freud's upper end stuff; but the pic shows "Avanti-Pro, which in my experience equates to Sears' old line "even at this low price", IOW GARBAGE - local Homeless Despot didn't have the Diablo versions of a couple different products, so I made the MISTAKE of bringing the Avanti stuff home - total waste of time, money, effort, etc...

Best luck I've had with mill scale on flat bar and angle is to start with a thin cutoff disk on 4-1/2" grinder, disk at about 45* angle and rapid movement across the piece til I get MOST of the scale removed - then a 40 grit flap disk to smooth out.

For larger projects (using tube) I found that getting the oil off went halfway easy by laying full 20' lengths of tube side by side on horses, spraying it down with Costco's de-greaser stuff then using a small floor buffer (mine's a "Gloss Boss" - I use the stiff brushes on the buffer and keep rotating all the tubes to get the other sides, then hose the whole thing down with water and let dry (summer sun helps :=)

Downside - don't wait too long before fabbing up and painting, or the NEXT problem is "how do I get the rust off"

The WINTER version of above: replace "summer sun" with a heat gun blowing thru the tubes; not ideal, but neither is "life"...

Best I can offer for an un-solveable problem... Steve

Exactly why i don't want to install this then spray it outside. Going to be hand rolling this asap inside.

 

[PCustoms]

The link text calls it Freud Diablo, which is Freud's upper end stuff; but the pic shows "Avanti-Pro, which in my experience equates to Sears' old line "even at this low price", IOW GARBAGE - local Homeless Despot didn't have the Diablo versions of a couple different products, so I made the MISTAKE of bringing the Avanti stuff home - total waste of time, money, effort, etc...

I grabbed the first link I found to show the idea....

That said if I don't plan ahead I end up with the Avanti as that is what is available locally. They work fine. The key with this type of abrasive is to keep it away from sharp edges.

 

[tarbellb]

3M woven pads is best bet- check Ebay for deals, or no BS Mcmaster

or get a backing plate for the grinder and 40/60 grit pads and go to town. In the end, its just PITA

HF link for reference- https://www.harborfreight.com/4-12-in-turbo-backing-pad-69776.html
+ Sungold/Mirka/Norton etc... pads, avoid HF abrasives when possible

https://www.amazon.com/dp/B00CH27Q6K/?tag=atomicindus08-20

 

[mark-NJ]

Hydrochloric (muriatic) acid will dissolve millscale rapidly. I use it for this purpose all the time before welding.

 

[bigguns69]

Right angle grinder and wire cup brush for cleaning material off. Been doing this for decades, and then use metal etching primer or direct to metal epoxy primer first with top coat final.

 

[HMCFab9]

Vinegar soak might clean it up.
Muriatic acid does it too, but it's a nasty smell, might etch your concrete floor & needs to be neutralized well.

 

[TMcCay]

I've used the vinegar soak method. It works very well for small pieces. For the larger pieces I am at a loss unless you use some kind of grinding/sanding method. But even with vinegar be sure to clean it off well, I use cheap window cleaning solution with ammonia in it.

 

[slowtwitch73]

This stuff works good: https://picklex20.com/

 

[mark-NJ]

For $180/gallon it ought to remove the scale and finish the project for me!

 

[csp]

No doubt mark-NJ, it's not exactly cost effective if your projects aren't the type you derive income from.

I have good luck with the paint prep discs, and if the parts fit in my blasting cabinet they get blasted to remove any residual that the discs don't get.

 

[slowtwitch73]

Yeah, sorry about that.. I didn't see the prices till later.

I used the stuff a number of years ago before they sold direct, and it was not that expensive. I checked the web to see if it was still around and posted it without noticing prices.... my bad!

It does work like the clappers.

 

[DrReid]

I bought a stack of Walter FlexCut mill scale removal discs.

They don’t clog or glaze, and they eat mill scale for lunch. I used a bunch when I prepped a lot of scrap plate in building my welding table. Alas, they’re not cheap, but they work!

 

[mikegt4]

I built an auto rotisserie (mostly out of sq. tubing) about 10 years ago but never painted it. Now that it is starting to rust I have been stripping it down to bare metal and painting it with Rustoleum. The tubing doesn't have a lot of mill scale compared to other shapes but I found that my Restorer sanding tool with 46 grit rust/paint removal drum cleans the metal in short order. I also use a Sunex surface blaster tool using the same type of abrasive allows me to get into tighter corners with the same results. So far the parts of the rotisserie that I have completed came out great.

I found out about the restorer in this thread, it works great on the sq. tubing.

Removing mill scale in cold weather

I've had good results using a white vinegar soak for a day or 2 to remove mill scale from steel tube and flat bar. The problem I have is that my garage shop hovers around freezing most of the winter. The cold inhibits the ability of the acid to do it's work in a timely manner. Been noodling...

www.garagejournal.com

​

 

[readhead]

You are trying to solve a problem you don’t have. Tube steel is milled using a different process than flat bar, angle etc. There is no mill scale. I have built literally miles of guardrail and lots of it to CDOT specs. Their specification requires “ clean to white “ before priming. In the case of tube steel that means removing any surface rust and/or wiping the material with a product to remove any oily residue from the production process.

 

[MoonRise]

If the steel was "hot rolled", then it has mill scale (to varying degrees).

Cold-rolled steel has no mill scale, P&O (pickled and oiled) has no mill scale (the "pickle" in the name means that they removed the mill scale with acid).

Lowes metal/concrete prep. Phosphoric acid based. ~$20/gallon

https://www.lowes.com/pd/Klean-Strip-Concrete-and-Metal-Prep-1-GAL/1001456348

Brush or spray it on.

Tightly-adhering mill scale is may be OK under primer and paint. Loose and flaking stuff is obviously no good.

But 'best' is to clean the steel to clean and bright and 'shiny' (aka to "white" condition).

(and if the steel is not bright and 'shiny', it was NOT "cleaned to white". Any rust or 'black' or 'gray' (which IS the mill scale, aka black iron oxide) means that the steel is not "white".)

Want 'industrial'?

Aerogreen AC-10 Rust Remover and Metal Prep | Sherwin-Williams

Aerogreen AC-10 Rust Remover and Metal Prep is a concentrated, water-reducible phosphoric acid and detergent blend specially formulated to both thoroughly clean and profile steel, galvanized metals, zinc, concrete and masonry/brick surfaces before applying a coating, stain or sealer.

industrial.sherwin-williams.com

 

[MoonRise]

ASTM spec for guide for paint inspectors is ASTM D3276. Flat out says that "clean to white" means clean it to BARE 'white' metal (via the reference sub-spec NACE SSPC-SP5/NACE No.1) via media blasting. No coating, mill scale, rust, oxides, etc allowed in this level of cleaning/prep.

One step 'down' from that level of cleaning and prep is "near-white metal blasting", which allow for some residual stains of no more than 5% surface area to remain.

 

[corn chip]

lately ive had several projects with hot rolled rectangle tube and plate. removing scale has been easy with makita right angle . heavy scale i use dewaalt 4 1/2 disks from ace ( theyre made in canada as a bonus). light scale i use 4 1/2 forney flapdisks of 120grit also from ace (made in turkey ).

 

[Kaizen]

I bought a stack of Walter FlexCut mill scale removal discs.

They don’t clog or glaze, and they eat mill scale for lunch. I used a bunch when I prepped a lot of scrap plate in building my welding table. Alas, they’re not cheap, but they work!

Those are not bad if they work. definitely cheaper then diamond wheels

 

[Kaizen]

You are trying to solve a problem you don’t have. Tube steel is milled using a different process than flat bar, angle etc. There is no mill scale. I have built literally miles of guardrail and lots of it to CDOT specs. Their specification requires “ clean to white “ before priming. In the case of tube steel that means removing any surface rust and/or wiping the material with a product to remove any oily residue from the production process.

yea we established that above. some of the steel was not cleaning and had different colors and i thought it was scale.