Painting a steel bumper - DIY notes & pics.

So I got a beautifully-welded steel bumper w/ swingout + jerry can mounts, and a set of sliders from 4xInnovations. All arrived unpainted bare steel. Time to paint!



CAVEAT LECTOR:
I had no prior experience using automotive-grade urethane finishes, acid prepping, or HVLP. Though I kinda doubt it, it's possible I've overlooked some critical step in my process which may result in early failure (peeling/bubbling etc.) of the coating. Look forward to comments/suggestions, particularly from auto-body / paint-shop professionals.

Overview of the process I used:

1. Cleaning/degreasing/de-labelling
2. Hydrochloric (aka muriatic) acid stripping/etching
3. Phosphoric acid surface conversion/steel prep solution
4. Final rinsing, drying, & cleaning
5. Primering (2 full coats)
6. Finishing (2 to 3 full coats)

Choice of finish:
I went with a corrosion-resistant primer + 2-part urethane finish coat system made by a small company called Mastercoat. (Mastercoat rust-preventing silver primer, Mastercoat AG111 satin-black finish.) Mainly on the strength of an extended-salt-spray rust test executed by an earnest youtuber.

Application method:
Cheap HVLP sprayer with a 1.8mm nozzle, driven off a little 2-gallon 1HP air compressor. (NOTE, the use of an inline air/oil catch filter is required when using urethanes or any moisture-sensitive finish.)

1. CLEANING/DEGREASING/DELABELING
Initial cleaning of the metal done with ZEP® Heavy Duty Citrus Cleaner, applied liberally with a trigger sprayer & wiped in/off with clean shop rags & paper towels. Then a thorough hose-rinsing. This removed ~98% of grease & grit, etc., but of course didn't touch the gray-to-black mill scale that comes on most hot-rolled steels.

There were a couple of large, tightly-adhered plastic UPS shipping labels on the parts. These came off with a liberal amount of GooGone™ Industrial Strength, soaked onto a clean paper towel.



2. ACID STRIPPING/ETCHING
--> It is best to completely remove mill scale from steel to allow a primer to properly & fully adhere to the base metal. This is most easily done with concentrated acid.

Mill scale is hard remove with mechanical methods. Nearly impossible to do well on complex surfaces, tubes with tight inside corners, etc. In a fabrication/industrial process setting, scale removal is generally done either in chemical dip-tanks (acids or solvents,) or via media blasting. I don't have either of those available to me, so I improvised an acid-stripping process by spraying the parts liberally with concentrated 31% hydrochloric acid (aka muriatic acid.)

Due to the large amounts of acid vapors this open-spray process releases, this method requires an open back yard or lot w/ at least 15' from structures on all sides & no neighbors any closer than ~50' downwind. Concentrated HCl isn't as bad as say, hydrazine or hydroflouric acid, but it's still very nasty stuff that requires respect & knowledge of basic acid-handling practices which are outside the scope of this write-up. If you don't know how to handle concentrated acids you are well-advised to spend the hours needed to self-educate before attempting to use them.

( Some have suggested in other threads that one could use milder solutions of vinegar, or even diluted acids, in a large kiddie-pool or poly-lined plywood-box immersion tank, but even a custom sized plywood dip tank needs huge quantities of expensive vinegar (think 20-30 gallons at $15+ per gallon,) and possibly a week per part (i.e. for the bumper, then the 1st slider, then the other slider) to accomplish. I didn't have $$ or patience for that & opted for far hotter acid that works in a matter of seconds to minutes.)

An acid-rated pump sprayer is, in my view, required for this method. It allowed me to apply acid quickly & controllably while keeping a minimum 3-4 feet standoff distance from the spray. There's no way to do this safely with a trigger sprayer. I used this one and was completely satisfied with its performance & acid resistance:



I used a total of about 2.5 to 3 gallons of 31% hydrochloric (muriatic acid), available from any pool-supply store at around $10-$15 gallon:



In my case, the ground beneath happened to already have black 4-mil poly tarp plus decomposed granite, a good makeshift catch-basin for the acid dripping down. But I'd have done it over bare soil, so long as it wasn't food-growing soil or immediately in a childrens play-area etc. HCl itself is easily diluted out by copious amts of water and quickly neutralizes itself against any number of mineral or organic compounds in/around soil. It is, after all, the major component of mammalian stomach acid and part of what comes up in vomit/puke from cats, etc.**

(**Caveat, I don't know what amounts of trace contaminants (such as heavy metals, etc.) may be present in swimming-pool-grade muriatic acid. Hopefully not too much. I figure it must not be too bad if they sell it ready to be dumped directly into hot tubs and swimming pools.)

Here is what the parts looked like before acid stripping/etching. (Sorry for the poor photo composition, it's the best I've got.) Interestingly, mostly it was the 2" square tubing that had heavy mill scale, but there was also considerable scale & HAZ discoloring on & around the welds:



Here are few shots of the bumper immediately after approx. 20-25 minutes of repeated spray-downs with 31% hydrochloric acid. This looks so clean that it might be mistaken for already being fully-coated with primer, but it's actually pure bare steel:









The saw horses pictured are made of aluminum, with plastic+steel leg hinges & locks. They did not suffer any damage from ~1 hour of exposure to 31% HCl / muriatic acid.

After a copious water rinse w/ garden hose ( >10 minutes, must rinse & dilute away the cleaning acid,) the moment the metal starts to dry out it begins flash-rusting. This is not a problem, first because I use a phosphoric-acid steel prep solution (OSPHO™) to mitigate flash rust, and second because the Mastercoat silver primer is specifically designed to be applied over thin, tightly-bound rust like flash rust. (NOT over heavy / flaky / loose rust.)

3. PHOSPHORIC ACID SURFACE CONVERSION / STEEL PREP

This step more or less arrests the flash-rusting reaction which occurs between wetted bare steel & the oxygen in the atmosphere. I'm not a chemist, but my understanding is that the mild phosphoric acid in the OSPHO™ prep solution converts iron-oxide (rust) to some kind of inert iron-phosphate (?) material which a) remains mechanically bound to the steel and b) does not undergo any further oxidation. A rust-stopper if you will.

--> The most important thing I've found when using OSPHO™ is that after it is applied liberally (completely wetting all surfaces,) the excess must be wiped off with a clean rag or paper towel. --> Leaving excess OSPHO™ on the parts leads to unsightly sticky polymerized drips that are difficult to clean off later. Wipe everything down to eliminate any visible traces of liquid/droplet OSPHO™ solution.

I let the parts sit anywhere between 2 and 12 hours, depending on my level of patience. OSPHO™ instructions say "overnight" but 2 hours seemed "good enough" to me.

Then thoroughly rinsed all w/ garden hose. Here's OSPHO™ for you:




4. FINAL RINSING, DRYING, & CLEANING

After another thorough rinsing, there was still some very light and uniform flash rusting. Not a problem as the primer is designed to apply directly over clean metal + tightly-adhered rust. At the point when the parts were ready to primer they looked like this. (Note that they were & must be absolutely bone dry, without any dampness in hidden inside corners et. Doing this outdoors on warm summer days achieves a rapid & complete drying.)

After all the water had dried, I used clean rags/paper towels soaked in pure acetone to do a final cleaning of all the large, easily-accessible flat surfaces of the parts. I didn't worry about anything I couldn't easily reach. I like pure acetone as a cleaner & solvent because it is aggressive, evaporates without a trace, is very low toxicity, and is not a carcinogen. (The human body creates considerable quantities of acetone internally as a natural byproduct of certain metabolic processes.)







5. APPLYING PRIMER

Save yourself some headache:
Be sure to protect all threaded holes, bearing races, and any other finely-toleranced surfaces during the coating process. Don't get primer or paint in them. I failed to do that and spent probably 2-3 hours carefully & gradually chasing all the coating out of every threaded hole by working fasteners partway in, backing them out repeatedly, scraping out flaked paint, etc.

If I had it to do over I would probably use plumber's putty (doesn't harden) to carefully fill all the holes before painting. Just running bolts into them before painting would risk the bolts getting seized by coatings that get wicked down into the threads.

----

A note before getting on to the actual work -- both of these Mastercoat products, the primer & the 2-part final coat, are urethanes, and as such they are extremely sticky. However, both easily & quickly dissolve & rinse away in pure acetone. I used lots of pure acetone as needed to clean tools, HVLP spray gun etc. between coats.

Everything is now ready for a first spray coat of primer. The Mastercoat silver primer is a 1-part product that comes in a can. The carrier liquid & the solids are fully-separated (like oil on top of peanut butter in a jar) and require at least 6-8 minutes of careful prodding, mixing, & stirring to fully combine.



I thinned the primer lightly with supplied Mastercoat-approved urethane-safe thinner, in this case Ethyl-3-Ethoxypropionate, supplied by Mastercoat also. To the best of my knowledge & study this stuff isn't "all that bad" as chemicals go. The MSDS seems rather tame. The thinner smells very strongly of -- pineapple:




(Note: Airless electric sprayer NOT recommended. I first used my expensive GRACO™ hand-held TrueCoat electric airless sprayer. This worked OK for the first coat but it sprays way too heavily & consumes a lot of material very quickly. I then used acetone to try to clean it out & ended up destroying most of the polymer seals inside the airless sprayer. It now needs a full rebuild before it can operate again.)

After destroying my airless electric sprayer, I bought a cheap and perfectly-adequate gravity-fed HVLP sprayer on Amazon for about $75. (Note, you must use an in-line air/oil separator filter at the gun inlet to avoid getting water droplets/vapor in the spray stream. Water in the spray will cause the finish to cure unevenly.) Combined with a 1/4" x 50' air hose, with my little 2-gallon 1HP air compressor regulator set to 60PSI, and an HVLP tip size of 1.8mm, I was able to get a consistent spray pattern.

--> HVLP sprayers of this size really want about a 15-gallon / 2 HP compressor at minimum for continuous spraying. Using my little compressor for this task resulted in a 90-95% duty cycle. Compressor is rated for 50% duty cycle, max. It started to get very hot so I had to take 5-10 minute breaks to let the compressor rest & cool down.





The primer went on without trouble, although with a somewhat bumpy texture. I think the texture is probably due to my small air compressor + cheap HVLP gun + lack of experience with this coating. Maybe more thinner, or less thinner, or 1 size smaller spray tip, or different air-pressure settings or HVLP gun settings, etc. might've made a smoother finish. But it's fine for my purposes, this is a bumper + sliders, not a clear-coat for a show-car body:










With the parts resting on saw-horses I had to coat 1 side, immediately clean out the HVLP gun + hopper, wait 6-8 hours, then flip the parts & repeat. Time-consuming, would be better to hang the parts from wires or a rack and to avoid this, but I didn't have anywhere large enough for 80-120 lb parts of this length.

Multiply this part-flipping by 2 full coats of primer and 2 coats of finish, it works out to 8 full cleaning cycles for the HVLP gun. I used nearly half a gallon of acetone for cleaning & flushing spray equipment between sessions.

Don't forget to coat brackets/fittings that are part of the mounting system:



6. APPLYING FINAL FINISH

After the primer was fully dry and hard to the touch (not tacky,) which is ~6-8 hours depending on ambient weather conditions, it is ready to receive finish. I waited a full 14-18 hours overnight to be on the safe side. I did not do any sanding of the primer, but did do another brief acetone-rag wipedown of the parts. (The cured/hardened primer does not dissolve or soften in acetone.)

The Mastercoat AG111 chassis-finish is a 2-part urethane-based product. The black comes in either gloss or satin finish. I chose satin finish but I'd say it's closer to semi-gloss than satin. That's OK, I'm sure road dirt & sun will take the sheen off in less than a year.

Unlike the primer, the solids portion of the 2-part finish came more or less completely mixed & needed little if any stirring on its own.

Note: Mix 2-part finishes carefully & accurately. I winged it on my first batch of finish, undershooting the hardener by maybe 20%, resulting in some nervous extra hours waiting/hoping for the surface to harden up. Supposed to take only an hour, if you don't use the correct amt of hardener it can take 4 to 6 hours. (If you use too much hardener, you may get bubbling in the finish due to excessive reaction rate & gas generation.)

Here's how the finish looked coming out of the HVLP gun going onto the bumper. The spray pattern looks quite rough, this first batch I didn't thin enough. But the AG111 coating has good self-levelling properties so the final cured coating doesn't look anywhere near this bumpy:

https://www.youtube.com/watch?v=BpptmxSmWsg

Here are a couple of shots of the AG111 coating right after (within 2-3 minutes) of spray, still fully wet:





And, after 2 to 3 coats and a full cure overnight, here are the finished parts:





Used a total of 2 quarts of primer and 2 quarts of finish for a bumper + swingout + 2 NATO can carriers + 2 sliders, with 2 to 3 full coats each of the primer and the finish paint. Though fully 30% of that was material wastage due to needing to clean out the gun 8-10 times during the process, doing experimental sprays & brush coatings on cardboard, etc. An experienced person working in a shop w/ the parts hanging from wires probably could have gotten it all done with 1 quart of primer and 1 quart of finish.

The Mastercoat™ instructions call for 5 to 7 days before the urethane finish reaches full hardness so I'll wait at least a week before mounting these to the truck. Passing that along for anyone who might be on a tighter time schedule & expecting to mount up their painted parts the next day.

I will update this post and/or thread with long-term notes on finish wear & corrosion, though these will be of limited value since I live on the west coast far below any snow altitudes. (No road salt/de-icers/sand out here, and winter rains are very clean / non-acidic.)

 

Reserved for future use.