Coating on square tubing?

[buening]

Sorry for a dumb question, but does steel square tubing (with joint/non-DOM) typically have a dark coating applied to it from the manufacturer? I bought some 2"x2"x1/8" tubing that had light surface rust and while taking the surface rust off there is a darker tough layer below it. I'm not sure if that coating is actually rust or something the manufacturer put on it, but a coarse cupped wire wheel on an angle grinder is having a hard time cutting through it. I can only break through it on the rounded corners of the tubing. Obviously grinding or using coarse grit sandpaper will cut through it, but was using the wire wheel just to knock the rust off.

The reason I ask is because I plan on using Rustoleum Professional Gray Primer along with their machine gray topcoat, and I wasn't sure if I should remove this coating down to bare steel or just prime over it. Thoughts?

 

[Wastedwagesracing]

You might be talking about Mill scale.

http://wiki.answers.com/Q/What_is_mill_scale

 

[GirlnAgarage]

You might be talking about Mill scale.

http://wiki.answers.com/Q/What_is_mill_scale

+1


On quick random projects I've left it on and rattle canned the paint. If you need a good smooth nice finish take it off to shiny. Same for welding.

Mill scale can be pretty tough. Some use angle grinders, some wire wheels, some flaps wheels, some acid, some sandblast - usually a combination depending on how you need the surface, how big the work piece is etc.

 

[Jack Olsen]

I'm a little doubtful on the WikiAnswers paragraph. Mill scale is the coating you're talking about. Some people (especially people working on very small projects) remove it prior to painting. But it's a lot of work to get it off. The idea that it will eventually flake away seems either incorrect or out-of-date to me. My wrought iron fence was painted more than two years ago, and is exposed to the California sun and sprinkler spray twice a week. There hasn't been any flaking away of the mill scale yet. Properly painted, I suspect mill scale will stay attached for a very long time.

Most steel is also covered with a light oil to discourage rust. You do need to take this off before you paint. Depending on the condition of the steel, it can be as simple as cleaning with a degreasing type of cleaner, followed by acetone. You might also need to sand and/or scuff the surface, depending on how long it's been exposed to the elements.

I've never seen mill scale flaking off of steel. Maybe steel used to be finished differently from the mill?

 

[MotoDave]

I like the red scotchbrite pads on a 3" die grinder for removing mill scale before welding

 

[lametec]

I work at a company where we manufacture stuff out of steel. None of our material is stripped of mill scale before painting. I haven't heard of paint coming off due to mill scale in the 14 years I've worked here. Steel that's painted a dark color isn't even primered first.

 

[buening]

Indeed, it appears to be mill scale. Sorry, I'm a bit new to metal fabrication! This is an 8' long workbench with 8 legs, so it is a decent amount of surface area to remove the millscale. I've went over the entire thing with the wire wheel, but as I said the millscale only got removed on the corners. I tried a 2" brown (surface conditioning, see pic below) scotchbrite pad on a die grinder and it took the mill scale off, but the disc only lasted one face of a single leg. I'd probably go through 100 discs and a lot of time removing it. This bench will be in a garage and won't see a ton of abuse, so it appears I'll just prime over the millscale! I just didn't want to prime over it, only for the paint to flake off in sheets and have to redo the whole thing over again. Thanks for the help everyone!!!!

 

[Mat Mobile]

Does mill scale affect the quality of a Weld?

I always had the impression that removing the mill scale produced a better weld (MIG) but there's so many variables (the biggest one being myself ) that it's hard to tell especially if you weld only as a hobby.

 

[lametec]

Yes, it affects the weld. Grind it clean before welding.

 

[Griff93]

I've found a sanding flap disc on a grinder is usually the best way to remove mill scale prior to welding.

 

[vpd66]

Does mill scale affect the quality of a Weld?

I always had the impression that removing the mill scale produced a better weld (MIG) but there's so many variables (the biggest one being myself ) that it's hard to tell especially if you weld only as a hobby.

Mill scale has a higher melting temp then the base metal. Yes you should remove it from the area you will be welding.

 

[csp]

Mill scale removal methods: http://www.pirate4x4.com/forum/showthread.php?t=945058

 

[buildmyown]

muriatic acid will take it right off but wil also make the steel flash rust and any other exposed metal rust quickly as well. Depending on how large a peice your working with you can put it in a container and let it soak or just use a scrub pad and the acid more labor but a lot eaiser then trying sand it off. The acid can be found at pool supply places and be sure to use the right safety gear.

 

[SWT Racing]

You might have A500 structural tubing versus mechanical P&O (pickled and oiled) tubing. The A500 is a little bit stronger, but for general use there's not much to worry about. The P&O tube will generally have a smaller fillet radius on the corners as well.

 

[cabriolet]

I agree with SWT, you more than likely have A500 tubing. Under the rust it is almost a black?