How do you get paint to stick to metal parts? (1 Viewer)

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I'm fed up.. I've tried painting metal parts countless times and have never been able to get it to stick. I've tried Rustoleum, duplicolor, etc. I've tried etch primers. It seems like I've tried everything and every time it seems like I'm left with a coat of paint that doesn't stick well and can be scratched super easily by metal (or sometimes even a fingernail)... Nothing like the factory paint I'm redoing.

Take for example this last batch I tried: These started as yellow zinc coated but were pretty rusty. I soaked them in a phosphoric acid solution and brushed all the rust off till they were bright bare metal. I then scrubbed any residues from the acid with a green scotchbrite and wire brush. I then washed them thoroughly with dish soap and THEN wiped them down with acetone till the rag was clean. The whole time I'm wearing rubber gloves so my skin's oils don't get on the parts. This last time I then painted them with POR15 topcoat, which says on the can that it's fine on bare metal. I let them dry three days, and when installing them, the new bolt heads pull the paint right off under them... Rubbing that area with my finger, even more flakes off.

WHAT GIVES????????????? WHAT AM I DOING WRONG?????? I see everyone else say "shoot it with rustoleum" like it's some simple task.

The only paint I've ever gotten to stick to bare metal is regular POR 15 and that's after the full marine clean and metal prep run around (basically what I did above).
 
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I've had really good success with POR15. Are you sandblasting as part of your preparation? I sandblast, then the POR prep to a "T" - Marine Clean, Metal Prep, then 3 coats as fast as the prior one gets a real good tack. Sounds like you have the cleanliness part down. Are you painting when it's warm enough?

From another thread talking about rust. I fabbed this skid plate about 6 years ago from virgin steel. Wouldn't fit in my sand blaster so just used various abrasives to give the paint bite. 6 years in Ohio salt and the POR15 is still mint.

Taco Frame.jpg
 
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Yeah, I've had good luck using actual POR15. That always sticks and is super tough. My problems are when attempting to use any kind of rattle can finish. Maybe they just don't adhere well by nature?

I'd like to figure it out though because I would like to be able to use a simple spray can finish on misc brackets, etc, without having to go through a full POR15 process.
 

GLTHFJ60

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What's your process for painting larger brackets? Surely you don't acid wash everything you try to spray-bomb.

I knock the dust off with hose, wipe with acetone, and spray. If it's bare metal, a self-etching primer is good before you shoot it with color.
 
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Yeahhh, I've acid washed everything I've painted so far simply because I've had ZERO luck not doing it and only marginal luck when doing it. Mostly though, I've done this to get rid of any rust.

Here's the vacuum valve bracket for my 60 which I just painted.. Overall the paint stuck pretty well (when following the steps outlined above), but if i drag my fingernail across an edge, it flakes off in that area.

Is that normal and to be expected.

PXL_20201207_235818958.MP.jpg
 

Krondor

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What temperature are you painting at? Too cold of a temperature and the paint will not cure correctly/ fast enough causing it to peel easily.
 

GLTHFJ60

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^^ Temp is very important, yes.

Post up a pic of this flaking. In general, paint won't stick to a hard corner nearly as well as a softened corner, so it may be expected. Can you get it to flake with your fingernail on your painted flat surfaces?
 
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^^ Temp is very important, yes.

Post up a pic of this flaking. In general, paint won't stick to a hard corner nearly as well as a softened corner, so it may be expected. Can you get it to flake with your fingernail on your painted flat surfaces?

Hmm.. Maybe that has an effect on this. I did pay attention to temp and kept the parts (and paint) at room temp before and after application. But, I did take them outside (45F) as briefly as possible to paint them. Perhaps some micro amount of condensation formed due to the temp change??? I didn't notice anything though.

Here's a pic where I scratched and edge and the surface with my fingernail.. It sticks on the surface, but nowhere near as well as factory paint.

PXL_20201209_192939956.MP.jpg
 

GLTHFJ60

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Hmm.. Maybe that has an effect on this. I did pay attention to temp and kept the parts (and paint) at room temp before and after application. But, I did take them outside (45F) as briefly as possible to paint them. Perhaps some micro amount of condensation formed due to the temp change??? I didn't notice anything though.

Here's a pic where I scratched and edge and the surface with my fingernail.. It sticks on the surface, but nowhere near as well as factory paint.

View attachment 2520325

That looks to me like it's not cured. A rustoleum enamel style paint will cure hard enough that you can't do anything to it with your fingernail. Not curing may be due to temp.
 
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Also make sure you are sanding the parts bare metal of existing paint to put small scratches in the surface to be painted so the new paint has something to adhere to. 45 degrees is too cold to have paint cure properly. I put my parts in from of a quartz light after painting to flash off all the solvents in the paint so it will cure properly.
 
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Agreed w/ all of the above that it looks uncured. But note, I am bringing them inside after painting to cure in a ~70 degree room.

I dunno.. Maybe next time I'll worry less on prep and more about doing the actual painting at higher temps.
 
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I've also read that rattle cans like it warm. As in put them in 90F water before you use them. I've done it but don't know if it helps. But try to have your workpiece and paint above 70 when spraying.

Or if you really want to put the issue to bed get a $10 Harbor Freight purple spray gun and 2 part automotive epoxy primer. Rattle can over that.
 
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Yeah, maybe moving to a full epoxy paint would be the best bet. I've also heard VHT Epoxy spray works pretty well.

For now, I was gonna paint my new Aussie air cleaner assembly but instead, it got dropped off at the powder coater yesterday 🤷‍♂️
 
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I never had a problem using this as a prep.

 
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I'm fed up.. I've tried painting metal parts countless times and have never been able to get it to stick. I've tried Rustoleum, duplicolor, etc. I've tried etch primers. It seems like I've tried everything and every time it seems like I'm left with a coat of paint that doesn't stick well and can be scratched super easily by metal (or sometimes even a fingernail)... Nothing like the factory paint I'm redoing.

Take for example this last batch I tried: These started as yellow zinc coated but were pretty rusty. I soaked them in a phosphoric acid solution and brushed all the rust off till they were bright bare metal. I then scrubbed any residues from the acid with a green scotchbrite and wire brush. I then washed them thoroughly with dish soap and THEN wiped them down with acetone till the rag was clean. The whole time I'm wearing rubber gloves so my skin's oils don't get on the parts. This last time I then painted them with POR15 topcoat, which says on the can that it's fine on bare metal. I let them dry three days, and when installing them, the new bolt heads pull the paint right off under them... Rubbing that area with my finger, even more flakes off.

WHAT GIVES????????????? WHAT AM I DOING WRONG?????? I see everyone else say "shoot it with rustoleum" like it's some simple task.

The only paint I've ever gotten to stick to bare metal is regular POR 15 and that's after the full marine clean and metal prep run around (basically what I did above).

You’re doing a lot of stuff wrong.

POR and most other paints are not intended for smooth shiny metal.

Paints only adhere to metal if the metal has an appropriate surface profile. This is accomplished by sandblasting or sanding. Etching primers can also assist with this, but etch primers aren’t going to perform as well as creating a proper surface profile. Additionally, most real etch primers require a mid primer coat between it and the topcoat.

Primers usually assist with adhesion. You’re applying topcoat paint directly to metal, which typically doesn’t result in the best adhesion or corrosion resistance.

You’re using rattle can paint. You’re never going to get quality results out of a hardware store rattle can. Not because of the can, but because of the inferior products inside.

Sharp edges will always hold less paint then their surrounding flat surfaces. This combined with inadequate surface profile is likely the reason why you can scrape off paint with your fingernail. You should always consider applying a first coat just to edges and then paint as normal. This won’t help with your adhesion issue, but will provide necessary corrosion resistance to the sharp edges.
 
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You’re doing a lot of stuff wrong.

So, without a blast cabinet, and short of sanding every piece I paint, what's the best way to prep the surface? I was under the impression that acid wash (50% phosphoric acid solution) and scuffing with something like a scotchbrite would be fine?

I understand a hardware store rattle can paint is never going to be as good as a true automotive finish, but I'd expect I'd get better results than I have?

So, moving forward: 1) scuff properly, 2) etch primer, 3) regular primer, 4) topcoat?

Thanks for the help everyone
 
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So, without a blast cabinet, and short of sanding every piece I paint, what's the best way to prep the surface? I was under the impression that acid wash (50% phosphoric acid solution) and scuffing with something like a scotchbrite would be fine?

I understand a hardware store rattle can paint is never going to be as good as a true automotive finish, but I'd expect I'd get better results than I have?

So, moving forward: 1) scuff properly, 2) etch primer, 3) regular primer, 4) topcoat?

Thanks for the help everyone

Scuffing metal will just polish it and make it too smooth for paint (which is the problem you already have). You need something a little more aggressive than a scotchbrite pad on metal. You need to put some “tooth” on the surface for the paints to adhere to. Nothing sticks to a smooth surface.

The last thing you want on brackets and hardware that will be prone to chipping is a multi-layer paint system. Thicker isn’t better. Keep it simple. Try to find a single primer and single topcoat paint.

Harbor freight sells blast cabinets pretty cheap. It’s a must-have if you’re restoring a car.

Compete rust removal and proper surface profile will give you the best results. Incomplete rust removal and insufficient (or too much) profile will result in premature failure. There’s probably a happy medium in there somewhere for you.
 
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I would get a blast cabinet in a heartbeat if I could fit it. My problem is space... No room for a cabinet or the big compressor needed to run it in my half of our two-car garage.
 
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Are applying several thin coats with enough time between coats for the paint to flash off. If you try to paint in one or two heavy coats the paint will not cure properly.
 
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Are applying several thin coats with enough time between coats for the paint to flash off. If you try to paint in one or two heavy coats the paint will not cure properly.

Yeah, definitely.. Last batch was 3 coats, each with 1+ hours between them (as per the paint's instructions).
 

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