Steel prep for welding/finishing

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This is going to sound dumb to most of you, but being new to working with steel, I don't really know the answer...When I first started practicing with my flux core machine, I was quickly running a wire wheel over the metal with a cordless drill and then running my test beads on it.  All fine and good.  Then I tried a sanding disk and found that I wasn't really removing mill scale before... I was just polishing it (if that makes sense).  All fine and good.  Now I better understand the "shiny metal" I want to see for welding.  My question is, let's say I want to build a welding cart.  I use vinegar or acid or sanding disk to get my spots down to shiny metal before welding.  Great.  Once i weld them, I can hit them with spray primer and later paint.  (not trying to win awards, just prevent rust).  My question is, Do I need to clean/sand the whole project down before I prime it?  Or how does primer do over mill scale?  In a lot of the videos after they weld something, they just hit it with primer or paint and keep going.  Just wanted to see if there's any issue with that?
Reply:It depends on the project and the desired finish. I typically use flap disks to get shiny metal where I’m welding. Then I cleanup any rust or loose mill scale. For a shop item that is usually enough before primer. If the project requires a better finish then I use the flap disk to clean up all the mill scale. Of course you may need to go to finer grits to blend or smooth any scratches. My powder coater recommends spraying with an Iron Phosphate solution before priming. He always sprays before he powder coats and claims that prevents any rust or mill scale flaking. I’m neither a chemist nor a painter, but I never have any problems with anything he coats.Burt _____________________Miller Syncrowave 250Millermatic 211Miller 375 Plasma Cutter Hobart Handler 120www.10FtDrillBit.comwww.MyWelds.com - pictures of my work
Reply:Let the mill scale wars begin.

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Reply:I just grind where I am welding if metal buildings.  AG welding I just weld over the scale. Let flux do it job.Dave

Originally Posted by Freebirdwelds

Let the mill scale wars begin.


Reply:For shop-use projects, I generally just wipe the whole mess down with drain oil after welding. PATINA!


Reply:I know this is a bit off topic, sorta.......................Many years ago, I got ahold of a bunch of steel from either Poland, or Russia.  A lot of it went into feeders for the cattle.  I believe I was in my mid 40's when I made that particular batch.  They've been out there, unpainted, for all that time, and there's been no really bad corrosion..............rust yes, but rust jacking/flaking no.Made another batch with steel either from the US, or Mexico.  That was probably around maybe 6yrs ago.  Every damn one of them is flaking like crazy.All of the affected steel is hot roll.......channel.  I've never figured it out, and I only had a sort of half baked answer from the LWS.  Something about the chemistry, but I forget what it was.
Reply:I agree with others that it depends on the finish you are trying to go after. I did a small metal project as an artsy gift recently and used a heavy knot wire wheel on an angle grinder to prep the metal. I burned all the mill scale off and got down to that "shiny metal" you are describing. After welding I did the same thing. However, the welding discolored the metal with the heat. That I did not work at to remove - I left it on purpose. After the parts were done I wiped down with Acetone and used Rust Oleum Clear Coat to prevent any rusting, yet leaving as close to the original "raw metal" appearance. So far so good. There are 4 spot welded and filed level "feet" on the base that got clear coated also - I presume over time if any area is going to start having issues its going to be those or around the few tapped holes in it.
Reply:I clean to shiny the weld area as most mentioned.   Concerning the rest - if you want a nice, 'clean' finish & paint job- then you'd want to flap disk whatever surface you are painting.  I think a solid, thick primer will cover fine and paint will sit fine - but I've found that if I haven't cleaned (flap disked) a surface either the scale or oily surface of the metal won't let the primer/spray paint seat nicely and I get a raunchy finish.  So - for me - any surface I want a clean, nice paint job on - I'll flap disc off the surface smeg....why don't you try it both ways on two small sections of metal - then go down the path you find the results most amicable for - it's your cart - assuming it's kept inside - wouldn't overthink it too much!
Reply:Oh, I forgot to say I always wipe the entire project with acetone before primer.Burt _____________________Miller Syncrowave 250Millermatic 211Miller 375 Plasma Cutter Hobart Handler 120www.10FtDrillBit.comwww.MyWelds.com - pictures of my work
Reply:Just the info I was hoping for.  Thank you all so much.
Reply:I'll some more info on the finish process.Spray cans need time to dry in between coats so follow the instructions on the can.It's usually ok to spray another coat within an hour BUT if you can not get another coat on within the hour then WAIT up to 48 hours until you spray another coat.The first coat has to dry completely.Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:Depends, can start at remodelers best friend,,,,, maroon Scotchbrite, which is the first box of stuff a painter should buy and all the way up to sandblast if you want and even blast the sanding marks right out of structural steel.  Then we scuff it with,,,, maroon Scotchbrite, blow the dust off and prime.   I am usually somewhere in the middle on most projects, some scuff, some sanding and some painting.  I recover lot of appliance steel and whle a guy has to scuff some paint to weld over the upside is the steel is well pre finished for free too, scuff and paint.www.urkafarms.com