paint prep for mild steel tube

Hey all--

potentially noob question here. I've got all my mild steel tubing cut up for my brew stand, and I'm itching to start welding. But I recognize that any wort of paint prep is probably going to be a LOT easier to do before welding, leaving only a 'last wipe' before putting paint on.

Soooooo, here's my dilemma. The steel obviously has a greasy coat on it, understandably to protect it from oxidation/rust. When I looked around on how to remove it, immediately I found lots of talk about mill scale. Sure enough, there's a layer of tough, black material on the entire exterior of the steel; I had assumed that it would 'rub off' with the oil. I put some elbow grease into it, and a couple hours later, and 2 drills in thermal overload protection, I had gotten through stripping a whopping 15% of the steel down to shiny metal. Tops.

Sooo...I went back to the interwebs, and I found a few posts from guys that stated that most square tubing (mine is 2x2x 11ga) is already 'pickled' to removed the mill scale, and then oiled for protection. Consensus seems that painting over mill scale is NOT ok, but that painting pickled metal is.

What's the skinny, folks? Is this mill scale or a protective oxide layer formed by pickling? Either way, do I really need to remove it? I have removed all of the unknown material near any/all of the weld areas. I am planning a flat black VHT flameproof paint, should that matter.

Thanks in advance!

Good question. When I started fabricating my single tier and was confronted with that nearly black coating, I perused a number of detailed build threads and noted that pre-paint prep usually included grinding down to bright base metal.

So I went at it with my angle grinder and 120 grit flap disks.

208' linear feet of 1.5" wide tubing surfaces and at least a dozen flap disks later, the whole rig shone like fresh stainless steel. And halfway through that nightmare I was seriously wishing I had welded it out of stainless.

I'm not positive that this was necessary, but my gut told me it was, and I usually respect my gut...

Cheers!

I have used a lot of this in past. Just weld it wipe it down with brake clean to remove oil and dirt. Scuff it with sand paper if you want not necessary though then paint. I've never had problem with paint not lasting

Day_trippr--you're clearly a person more strong-willed than I--I gave up after only about 2-30 linear feet, of 224 total, 2" wide . But that selfsame frustration is the reason I'm asking now!

tel72--Thanks for the input. I've seen a bunch of recommendations to use degreaser, acetone, break cleaner, alcohol, mineral spirits, etc etc to get the oil off. I found both degreaser and acetone to work well with getting the oil off, but the degreaser needs a swipe with acetone anyways to get any remnants off. But most such recommendations also say to get it clean down 'to the metal', so you can see the confusion that results.

Any particular grit I should be using for scuffing? I have power pads running anywhere from 36 to 400.

Any other advice/input would also be appreciated!

You don't really need to remove the mil scale. I'd say it's more important that you clean the oil/grease off with a decent degreaser and then use a good rust preventive primer.

Thomsahp--when the area will be exposed to high heat, what primer can you use? I didn't see any primers that can withstand the temps.

Or perhaps, you're referring to a phosphoric acid type converter that will effectively act as a 'primer'?

Oh, well I go electric so heat was not an issue for me. They do make some high heat primers though. I think rustoleum makes one. The only areas you would need to worry about that would be the area right around the burner. The rest of the stand you could use normal primer and paint. I would think that the area near the burners wouldn't do to good whatever paint you use.

I just use a light grit to remove any build up the brake clean didn't get or any surface rust that's starting I use the same type metal to build treestands that are out in the elements and the paint holds up really well as far as heat primer I'm not sure I have used the rustolium heat paint like for grills and it works but depending on how you mount your Brunner and that amount of heat being transferred to the metal not sure if anything will last a long time. I say drink couple beers build your cooker and don't stress over it to much

Mind you, I haven't tried this yet, but I'm also in the middle of a brewstand build. I've heard that Muriatic acid will dissolve the mill scale with minimal effort on your part. My plan is the build the stand then when I'm ready to paint, use a pressure sprayer (you know, one of those bottles that you hand pump) and wash the stand with the acid. Then hose off with water and dry it, then paint it.

Don't wait to paint it though, because it will flash rust real quick. Then you'll have grind the rust clean.

Freak coincidental refresh a minute after your post, Khomburg

I'd be very, VERY careful about spraying muriatic acid, better know as hydrochloric acid. I'd recommend completely sealed goggles, face shield and a respirator at the very least.

That said, I've heard the same thing about it's wonderful properties. Indeed, when I was referring to 'pickling' earlier, that's what most folks were referring to as an agent. But in pictures I saw, it didn't leave a shiny coat, but a dull grey coat, which is very similar to what I have on the steel I have, now that I've gone and thoroughly degreased it all.

You don't have to remove all the mill scale. I would however remove the mill scale at the areas where the welds will be. You will get a cleaner stronger weld if you do.
That mill scale will actually prolong the formation of rust. Removing the mill scale from the entire surface is a waste of time, and will cause the metal to rust even quicker.

As I pointed out earlier, dmfa200--I'm not entirely sure it _is_ mill scale, which is part of my question to the collective .

And as I mentioned, I did grind clean all of the weld areas. Even though I'm stick welding, it's always a good idea to clean up your weld areas .

I don't know of any primer that is rated for high heat. I'd recommend picking up the exhaust manifold and header paint from an automotive store and read the prep recommendations on the can. If you do use low temp primer, you'll be wasting money on the high temp paint.

Thanks, Bobby_M. As noted, I am using the VHT Header Flameproof paint, rated to 2000ºF .

Believe me, I will be using a respirator and goggles when I do the spraying. I'll also be doing outside and on day with very little wind.

What I've heard is that, yes the mill scale will prolong the formation of rust, but over time mill scale will begin to flake off and those exposed areas will begin to rust. My feeling is that no matter how nice you apply the paint, it's only as good as the prep job you do before hand. I'm using the VHT flameproof paint also. They say that it doesn't require a primer coat, but yet they sell what they call a primer. In reality it's just their white paint. I bought 4 cans of both the primer and a color, so I'll be doing both anyways. As for the finish after the acid spray. I think a dull gray would be the proper finish for the first coat of primer. You don't want to be painting a shiny smooth surface anyways. Your surface should have some tooth to it so the paint has something to bite into and hold onto.

I used acetone to wipe off the oil and painted right after that (didn't even sand!)
Zero problems with my paint so far.

I however designed my stand so no painted areas are near flame, so header paint wasn't needed. I just primed and painted with some decent paint.

When I went to Auto zone, I didn't see a primer rated for the temps, and the paint didn't call for one, so I'm not too worried there .

I've heard that if you neutralize the acid with a base, like baking soda, the metal won't rust very quickly.

Smart move, krazydave. I'd like to make a stainless steel insert to belt protect the frame around the burners, but getting the stainless will prove to be a challenge here

Having cleaned several thousand feet of steel, I got to say simple green, spray bottle and a rag is one of the easiest and safest ways to go. You want to go with a higher concentration of simple green, but it works a charm.

There are several brands of high temp spray paint out there, look for grill paint or you could try for an epoxy paint.

In the past, if we were worried paint would not stick we would use a spray adhesive, like spray 77, for mist coat and e then spray over that. You want a very light hand or it will affect the finish.

Little late to the party, but thought to post anyway. I am planning to work with 2x2 11 ga this week. i intend to power wash steel after brake cleaner. Let a small film of rust form as proof the the oil is removed then use Over Rust from Sierra Industries. At 92 dollars a gallon its not cheap but it goes on like water thickness and dissolves rust and binds to the steel like nothing else I've seen. Then I plan to primer and paint. I'm a farmer, not a welder or painter, sooo... Fertilizer Dealer Supply carries it if you're interested.