prep for paint info needed

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I'm not sure if this is the proper place for this but I figure it might be as we all like to build and paint metal projects.My question is, what is your (proper) procedure for prepping metal to paint? The stuff I get has some sort of blue/black almost scale on it, I usualy hit it with a flap disk and then maybe a palm sander then wipe down with acryliclean, prime and paint.What all do you guys do? Am I getting the wrong steel?thanks,dave
Reply:I think this depends on the intended use of the material. they right way to do things is to remove the mill scale and paint with an epoxy paint intended for steel. but this is far from the only way steel is painted.
Reply:The mill scale should be removed in order to obtain a decent painted finish, as well as insuring that the paint doesn't flake off afterwards. By far, the best option is to sand blast the pieces if they are large, or are assembled so that you can't get inside corners if parts are already welded. If you grind the surface, you have to use fairly course abrasives and change them frequently to prevent glazing.
Reply:For *New* 2-1/2" x 2-1/2" x 1/4" HSS (HRS, A36) is removal of the mill scale optional, prior to epoxy primer and top coating?  Was planning to clean (e.g. mineral spirits, acetone) all surfaces, hit lightly w/ a fine grit flapper wheel, apply primer, and then paint. All preparation to occur before any cuts and/or welding.  Paint type/manufacturer TBD, but color will be matte black.  Not building an Armored Personnel Carrier, but desire longevity and really searching for a pragmatic and financially responsible approach.  Item will be semi-stationary (garage) in a non-maritime environment and will subjected to standard shop practices (ie. fabrication, welding, grinding, plasma cutting).  Would be cool to hear from anyone who opted to leave the mill scale intact on new steel and, for whatever reason, regretted that decision post-painting.To Remove, or Not To Remove.  That is the question.
Reply:When I get home with new steel for a project, I lay it out on saw horses and run a wire brush cup over it and wipe it down twice with mineral spirits. Then I apply a goat coat of Rustoleum for rusty metal primer to it. When I cut the metal for welding I just buff of the primer where the weld is going to be made. After the days welding is done, I clean the weld areas and prime them. After the the project is complete, it gets a second coat of the primer, then the paint goes on, usually 2 coats at least. It's way easier than leaving it bare steel until the project is done before all the cleaning begins and you have to try to get into all sorts of weird angles and nooks and crannys to buff off rust and mill scale. Also your hands and gloves stay cleaner during the build with the primer on everything instead of mill scale.
Reply:@ Bistineau - makes good sense.  Was thinking along those lines, BUT was planning to final paint all members prior to welding.  Well, that probably not good......because filtering/clamps ops during welding may potentially mar the final painted surface.  Primer ok prior to welding; final paint application prior to welding, not so good.Therefore, will adopt your protocol above.   Mahalo loa nui!Last edited by ManoKai; 02-24-2013 at 07:28 PM.
Reply:Originally Posted by ManoKai... was planning to final paint all members prior to welding.  Well, that probably not good......because filtering/clamps ops during welding may potentially mar the final painted surface...
Reply:I don't remove tight scale.  I wire brush, acetone wipe, use a good primer, and a expensive urethane.  Sometimes I use an automotive "self etching" primer and automotive paint.  Rustoleum urethane mastic is my favorite industrial paint, but it is close to $150 a gallon.  Their "rusty metal" primer, also can make a finish coat, which most primers won't. Imron (automotive polyurethane) is getting up towards $300 a gallon, even though it is bulletproof. The mastic seems expensive, but I paint all my custom rental party boats with it.  Once they are done right, I usually don't have to redo them. I would say it is as good as powder coat. I have painted commercial playgrounds, skate park fencing and other high abuse areas with it. I am painting two new double deck, waterslide, partyboats right now with a different, cheaper polyurethane from PPG (pitthane).  It seems tough, but time will tell, especially when it comes to gloss retention. Epoxies are possibly tougher, but chaulk in the sun. The polyurethane is mixed with a hardner but is slightly more flexible than epoxy, taking abuse and chips a little better. When you calculate labor of repainting, the expensive urethanes aren't as bad. My projects get a lot of abuse from the public.   They do make a "weld through" primer, but I have never used it.
Reply:@ blackbart - good information. Thx.  Those high-speed coatings cost more than my water jet cut 1/2" steel top and 1/4" HSS. Ha!  When I build that APC, will rip those paints.Last edited by ManoKai; 02-25-2013 at 06:56 PM."Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:Originally Posted by ManoKai@ Bistineau - makes good sense.  Was thinking along those lines, BUT was planning to final paint all members prior to welding.  Well, that probably not good......because filtering/clamps ops during welding may potentially mar the final painted surface.  Primer ok prior to welding; final paint application prior to welding, not so good.Therefore, will adopt your protocol above.   Mahalo loa nui!
Reply:Sometimes I will etch with a phosporic acid, if the paint needs a rough surface. I'm not convinced on it yet.  But I always insist on quality primer and paint.  If you go automotive, your finish coat is done that day.  I primered (dupont epoxy) and finish coated (Nason acrylic enamel) a commercial church playground in 1990's.  I have not had to ever redo it, It was better than the powdercoat back then. And believe me, those kids could break an anvil!
Reply:Oh, in addition, that 1990 playground was mainley built with black pipe, so I did have to wirebrush the black laquer coating off, but it is still looking as good as any other powdercoated project.
Reply:If it doesn't need to be a showroom finish (more like "workshop" finish), I just let it rust to give it some tooth, them prime with Rust Oleum rusty metal primer, then paint. The paint sticks like the hammers from he|| that way
Reply:In a perfect world the best way would be to remove any and all rust,oil,scale or other coatings. All pits and crevices etc need to be cleaned or even smoothed out as well. Sanding the parts with progressively finer sand paper to the max 320grit..spray with epoxy primer, then high build primer depending on how nice you want the finish... sand high build with 320-400 even up to 600grit then clean with wax and grease remover... and paint... if you want mirror finsh...sand paint smooth with 1000-1200g, then 2000g, then 3000g then buff and machine polish... Sand blasting is always an option...but remember that it will distort the lighter gauge metal, but still should be sanded smooth before priming...Powder coating is probably the best, most durable coating available...Lincoln pro mig 180Lincoln Square Wave Tig 300/wp 20/home built water cooler Victor, Purox, Harris, O/A welding/cutting setupsVintage Craftsman drill pressVintage Craftsman/Atlas 12"x 36'' lathe7''x 12'' w/c band saw Everlast 140 st
Reply:I have seen some horrible and then perfect powdercoats.  There are many variables, including the powder product itself. They are usually great, but not always.  I have just found the rustoleum 9800 urethane mastic to be equivelent in durability. They sell is as a direct to metal (no primer) product, but it is better over a primer.  Most of my projects are way to big to go in an oven though. I never thought about using the rusty metal primer over rusted metal, then finishing, interesting.  I use that brand as a primer/finish coat on all my marinas. My projects are subjected to all the abuse and vandalism that the US public can dish out. Imron has always been best, but it is almost out of everyones price range now.  One foolproof, extremely durable finish is Zolotone.  It hides many mistakes!   It is used on most boat interiors, usually applied with a cheap $30 home depot pressure type gun.  It's almost impossible to screwup and very durable.  It has to have a good primer though!