Bare steel paint prep

UPDATED: SKIP THIS THREAD, I POSTED A COMPLETE START-TO-FINISH DIY HERE:

https://www.tacomaworld.com/threads/one-way-to-paint-a-steel-bumper-diy-notes-pics.836279/

Original post:
Prepping a steel bumper for DIY painting, planning to use a steel-specific primer + 2-part epoxy final coat from MasterCoat™.

Obviously the best steel prep is absolutely 100% clean bare metal, achieved by submersion in a hot acid tank for 30-60 minutes, and/or sandblasting. I don't have a way to sandblast or to DIY a full acid submersion. Would take waaaay too much acid to accomplish.

So I sprayed 31% HCl (muriatic acid) over it, multiple times over 30 mins, and got to about 98% surface cleanliness. (Here's my full post w/ more photos from that process.)

 

Wire wheel to get the loose scale. If you use the truck the bumper will take a beating and you will be regularly touching up the paint.

 

Mechanical methods (flap wheels, wire wheels, die grinders) are a no-go here due to the size & complexity of the parts (lots of curves, hidden inside corners, etc.)

Going to try additional coats of 31% HCl, this time using a roofing torch to pre-heat the entire bumper + sliders to ~140-160ºF before applying. Strong acid is good, strong hot acid is even better.

This full-frame shot shows how clean a gallon of 31% muriatic acid (HCl) sprayed over ~20 mins got these parts. Fantastically clean for pratically zero effort compared to manual methods. I'm just wondering if I need to chase the last tiny 1-2% detail spots.

 

The one picture with the mill scale is easy enough to get with a wire wheel. Just get the easy to flake off scale. Assuming it's a truck, not a show car. Its paint ready.

 

That's just an example of one of probably 40-50 other small very small spots in many different locations, some basically impossible to get at with mechanical methods.

I'll be chasing this with chemicals & will update this post with my progress.

 

the main issue is impact damage. the millscale will flake off with a direct impact. if it's never going to see a direct impact on the spot, it's not likely to shift, and the paint will be more than enough to hold everything together for longer than your interest in the project.

 

Best thing to take of mill scale that's hard to get to would be soaking overnight in concentrated vinegar and water. I know that seems impractical here, but a kids wading pool would work if you're really wanting to get it off. I buy 1-gallon bottles from Amazon for around $20 and soak smaller pieces of metal in Tupperware containers. You can dilute at least 50-50, maybe more, and still get results.

You have to clean off the solution as soon as you remove it from the soak. Otherwise, it will start to rust.

Or maybe media blasting but I have no experience with that.

 

No, the steel parts are too big. I'd need a 6' kiddie pool filled at least 6" deep, and at that depth I'd have to mess around with flipping the pieces over for full coverage. That'd be 100 gallons of liquid, at a 50/50 vinegar solution that'd be 50 * $15/gal for vinegar (figure a quantity discount) = $750 for vinegar. And then the problem of trying to -- save it?! In (2) x plastic 55 gallon drums?

I guess I could fab up a plywood box of 72"L x 12"W x 12"T, line it with 6 mil poly plastic, and get full immersion. That would require only ~40 gallons of liquid (20 gals vinegar) but that's still $400 for vinegar and nowhere to store it for later use.

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Instead I'm going to get another couple of gallons of 31% HCl (muriatic) acid at $10 a pop.
This stuff is powerful, fumes immediately on contact with air, definitely at least a Class II FAFO substance. (Official Federal regulatory parlance.) The cleaning I've gotten done so far, basically a 97% clean, was done with just a single gallon, with no cleanup or disposal /storage issues to deal with. (Just neutralized what was on the ground with baking soda, then dilute w/ 25-30 gallons of water from garden hose.)

This method is only viable in an open back yard/field area at least 15-20' from any building and minimum 50' from neighboring lots. Doing it in any semi-enclosed space, even a car-port, is asking for respiratory trouble.

 

Evapo-rust is pretty good for pre-paint prep.

That kiddie pool, some washing soda and a 12 v battery charger could get you an "anodic clean".

Me? I'm into phosphoric acid conversion for ferrous metals.

 

Interesting, hadn't thought of trying an electrochemical process.

Def. will be using OSPHO™ (phosphoric-acid-based) surface-converting prep solution after thoroughly neutralizing & rinsing the HCl off the steel.

 

I keep an assortment of "old iron" strung on wires, a plastic trash can , and a box of Arm&Hammer washing soda around the shop all summer when I might be doing some steel work. Certainly a bumper is a physical challenge. The kiddie pool would be an option.

Getting the power density to a level that would be classified as productive might takes a bunch of amps, but when things go slowly, even overnight is not a risk. (Scotchbrite to remove any smudge....)

 

What voltage setting would you use, and approx. how many amps of current for a steel object the size of an all-steel Tacoma bumper?

Baking soda is very cheap & convenient to work with. No doubt a long electrochemical soak is going to clean more thoroughly & deeply, into microscopic pores etc., than a thin coating of acid rinsed over the part for 20-30 mins.

 

Do the best you can, but with a bumper surface area, I'm sure 30 amps (at 12V) would not be too much. Sometimes a battery can be used to give the high currents to speed the process.
But ultimately, the anode area will define the current flow.

Re-bar works, hog pen fencing works, sheet steel works. You can be creative.

Use an amp clamp, and look for bubbles ;-)

 

Didn't pursue any electrochemical methods, way too much trouble to set up a dip tank.

Another 20-30 minutes with a pump-type acid sprayer and ~2 more gallons of 31% HCl (muriatic acid) got the parts to literally >99.9% clean. I didn't bother doing any fancy stuff like trying to pre-heat the parts with a roofing torch. (It was very helpful to let the gallon acid bottles and the steel parts themselves heat up in the warm summer sun.)

I can't overstate how satisfied I am with this cleaning process & results. The acid prep got the parts far cleaner than any amount of wire-wheeling, sanding, scrub-brushing could have, with 20 times less effort. The downsides to the acid process are that it requires a fair amt. of open space, a special-purpose sprayer designed to handle high-strength acid, and the diciness of trying to breath-hold and guess which direction the breeze was turning to avoid inhaling too much acid vapor.

Looking at these photos (below) without other context, I would think to myself, "primer job looks pretty good, ready to paint." But that's not primer, it's all pure clean steel, still wet after the acid was water-rinsed away:





I then applied OSPHO™ steel prep solution with a trigger sprayer, wiped off the excess (very important), let the parts sit for ~2 hours or so in hot weather / direct sun, then thoroughly rinsed it all off with plain water again. Note, this goes against the OSPHO™ instructions, which are to let the parts sit overnight, but I was impatient.

This is what the parts looked like after completely drying, and a brief wipedown with a clean towel soaked in acetone. The flash rusting is all very thin and "tightly adhered." The metal primer I am using, MasterCoat Silver "slim" (their sprayable formulation) is specifically designed / listed to be applied over light flash rust / tightly-adhered rust:







Then did some test applications of the primer with a chipping brush on the underside, let it dry / cure for 3 hours, then did a 2nd coat with a brush and let that dry overnight to check for adhesion & make sure no issues with bubbling etc. The brush finish is ugly, this product is really best applied with a sprayer.







I have a Graco™ TruCoat Pro II hand-held airless sprayer which I used for the full primer application. Here's what the sprayed-on primer looks like after 1 coat, still wet. The primer instructions specifically say use "minimum of 2 coats," and you can see why here as the first coat has a very fine orange-peel texture appearance. I will touch up the under-applied areas in corners, etc. with a brush later on, after the 2nd spray-on coat is dried/cured.




Here's what 2 coats of sprayed-on primer look like after curing:






Primering still-in progress, last phase will be spraying with MasterCoat™ AG111 two-part (epoxy-type) matte black paint. Will post further photos when that is done.

 

Whoa! Nice work!
I primed and top coated the sliders I put on my Taco.
The winter road sand and salt really takes it's toll on the paint finish.

I'm thinking roofing tar is my answer! I wonder if I can get a color top coat to stick?/////// ;-)

 

Tar might be effective anti-corrosion but could be problematic on your shoes/carpets..

It's a bit funny I'm taking all this trouble to do deep prep & use one of the better-quality corrosion-preventing paint systems out there seeing as how this truck lives in north-western california, below any snow levels, so there is never salt on the road and it is mostly kept garaged. Vehicles generally just don't ever rust out here, even 25-year old vehicles stored outdoors. It rains plenty in winter but rainwater is pretty non-corrosive on its own.

I figure this way I'm prepared in case I ever move to a rust-belt state or alpine zone. Or want to use this as a selling point to a potential rust-belt buyer. Same reason I plan to start DIYing my own body/frame undercoating every couple years.

Here's a review + test of the paint system (primer + epoxy-type top coat) I used. At the end of his exhaustive video he concludes that the MasterCoat™ silver primer + AG111 matte black 2-part epoxy paint is one of the 2 top performers. I didn't want a RaptorLiner finish (the other top pick) so I went with the MasterCoat.

https://www.youtube.com/watch?v=TABWPHutk-A

 

Whoops. Destroyed my nice Graco™ TrueCoat airless electric sprayer. Stupid mistake. Should've been spraying this product through a cheap $60-$80 gravity-fed HVLP air spray gun.

I left the coating in the airless sprayer, fully primed, while waiting for the first spray coat to dry/cure. It was fine sitting for about 1-2 hours in the morning before re-spraying, but waiting 6 more hours till late afternoon on a warm day was a mistake. It's clogged beyond recovery & my attempts to use pure acetone to degunk it internally have destroyed several of the petroleum-based o-rings that seal various critical parts.

Pot life (how long it stays liquid) of the MasterCoat™ silver primer product in an opened-but-resealed container was fine (weeks,) but I guess there must be more pathways/channels for air/water vapor to get into the small parts of the airless electric spray feed/pump system to beging the curing/hardening process.

Painting to-be-continued in 3 days or so after I get new equipment... live and learn.

I ordered this cheapie & will run it (slowly) on my 2-gallon 5 SCFM @90 PSI air compressor. Cheap but hopefully effective enough for this rough application. It's a bumper after all not a hood needing a clearcoat.

 

i don't know if it's still the case, but i used to know a few guys that did vehicle painting on the side, and would use harbor freights cheapest HVLP gun. they all said it worked great, and was cheaper to toss and use a new gun for every job than it was to justify paying for a really nice gun that required meticulous cleaning after every use..

 

I don't mind meticulous cleaning on guns / setups that are designed to be easily & fully disassembled for maintenance. An electric airless sprayer really isn't. In this particular case the MasterCoat gray primer cleans up very easily with pure acetone, as long as the primer hasn't started to harden up yet. I'm thinking a cheap HVLP gun will handle acetone fine, unlike the many seals in the electric airless sprayer, some of which require complete disassembly of the liquid/spray unit to get at.

In fact I was going to drop a bit more $$ on a nicer Eastwood™ brand HVLP air sprayer, but that wouldn't arrive until July 1st. This primer says it's best to recoat within 48 hours, else sanding is required. Really want to avoid sanding so I'm going to recoat within 72 hours and call it good.

 

Until the job takes more than four cans of paint/ primer.. Rattle cans are the way!

And there are outfits that will "rattle can" just about any paint. Just not two part/// ;-)