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Old 04-26-2013, 02:30 AM   #1
RadicalEd
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Default 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!

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Old 04-26-2013, 05:45 AM   #2
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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!

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Old 04-26-2013, 12:33 PM   #3
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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

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Old 04-26-2013, 01:20 PM   #4
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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!

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Old 04-26-2013, 01:46 PM   #5
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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.

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Old 04-26-2013, 01:50 PM   #6
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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'?

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Old 04-26-2013, 02:37 PM   #7
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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.

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Old 04-26-2013, 03:19 PM   #8
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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

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Old 04-27-2013, 04:19 AM   #9
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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.

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Old 04-27-2013, 04:25 AM   #10
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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.

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