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1G Rust returned, what did I do wrong?

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XC92

Proven Member
1,561
356
Jul 22, 2020
Queens, New_York
This is in reference to my first attempt at derusting and painting some parts when I started restoring my '92 Talon two years ago. I did my best to remove all rust from the rear control and trailing arms and calipers until it was as close to bare metal as I could get it, using mostly wire wheels on a drill and impact driver and rotary tool.

I didn't have a die grinder back then which would have helped speed things up enormously, and have since purchased the Milwaukee M12 right angle die grinder which I can't recommend enough. Then I cleaned it with brake cleaner and alcohol, and then applied Rustoleum caliper paint to the calipers and rust reformer and auto paint to the arms. Looked great...for a while.

Two years later and the rust came back, mostly under the paint but also showing in some places. So I removed all the paint and rust again, this time with the die grinder and rotary tool, cleaned it with Super Clean degreaser, rinsed it, prepped it with Zinc Phosphate-based Metal Prep, rinsed, let it dry, then applied POR-15. This was just a week or so ago so there's no way to tell if it worked this time.

What I'm wondering is why the rust came back after my first attempt. Was it because I:
  1. Cleaned and degreased with brake cleaner and alcohol instead of water-based degreaser?
  2. Didn't treat with phosphoric acid-based prep?
  3. Used Rustoleum spray paint and rust reformer instead of something more impermeable and scratch-resistant?
  4. Something else?
I'm just trying to learn as I go. I suspect that my big mistake was not treating with phosphoric acid and assuming that the rust reformer or paint would protect against future rust, and my second mistake was using permeable and not very tough paint.
 
I'm learning the hard way that rust conversion primers are really neither. They're quick fixes but not long term solutions. I think the most essential step they skip is phosphoric acid treatment to neutralize whatever rust remains and etch the bare metal for better topcoat adhesion.
 
Spray paint is porous, allowing the underlying metal to be exposed to air, enabling the rusting process. Por15 is not porous, air can not contact the metal, thus rust can not happen.
I wonder if had I treated the bare metal with phosphoric acid if it would have helped. In any case I'm doing it this time since it's part of the POR-15 process. Subframe, arms, sway bar, diff support, calipers, a few other parts, all getting the POR-15 treatment. I'll probably also rebuild the prop shaft soon and POR-15 it too.

And eventually I'm going to overhaul the exhaust system from manifold to turbo to pipe/cat/muffler, but instead of POR-15 I'll use the KBS high heat version, which goes up to 1500F compared to 1200F for POR-15. I hate rust. HATE it.
 
This is in reference to my first attempt at derusting and painting some parts when I started restoring my '92 Talon two years ago. I did my best to remove all rust from the rear control and trailing arms and calipers until it was as close to bare metal as I could get it, using mostly wire wheels on a drill and impact driver and rotary tool.

I didn't have a die grinder back then which would have helped speed things up enormously, and have since purchased the Milwaukee M12 right angle die grinder which I can't recommend enough. Then I cleaned it with brake cleaner and alcohol, and then applied Rustoleum caliper paint to the calipers and rust reformer and auto paint to the arms. Looked great...for a while.

Two years later and the rust came back, mostly under the paint but also showing in some places. So I removed all the paint and rust again, this time with the die grinder and rotary tool, cleaned it with Super Clean degreaser, rinsed it, prepped it with Zinc Phosphate-based Metal Prep, rinsed, let it dry, then applied POR-15. This was just a week or so ago so there's no way to tell if it worked this time.

What I'm wondering is why the rust came back after my first attempt. Was it because I:
  1. Cleaned and degreased with brake cleaner and alcohol instead of water-based degreaser?
  2. Didn't treat with phosphoric acid-based prep?
  3. Used Rustoleum spray paint and rust reformer instead of something more impermeable and scratch-resistant?
  4. Something else?
I'm just trying to learn as I go. I suspect that my big mistake was not treating with phosphoric acid and assuming that the rust reformer or paint would protect against future rust, and my second mistake was using permeable and not very tough paint.


1. Anything water based still leaves a small amount of moisture behind so this can cause issues obv with not having the best base to adhere to when moving on to the next steps.
2. From my understanding, the metal prep etches the metal as well as leaves a zinc coating behind which is a rust preventative of course. Just like the oem bolts etc that are zinc plated.
3. I agree with what previously was said that spray paint is not forever and permanent. I only did the seams and laps of metal joints/pinch welds just to be sure in case there was a minute amount of rust or whatever behind, so that it would be neutralized. Then covered that with the other processes.
4. You could save yourself sooo much time, effort and headache later on by just taking the parts to a good powder coater that will use a chemical bath to fully strip the parts to bare metal, fully cleaned and then coat them. Powder coats if done right last a really really long time as long as they don't get scratched and dinged and its not really that expensive anymore. Hell you could even por15 over the powder coat and then it is going to last forever. Its just getting the metal absolutely 100% cleaned that is critical. Take this from someone as you have seen has done all the above with wire wheels, por15 etc haha.

Remember as well, por15 is great if the surface is 100%.. if not then its also great at trapping and hiding worse rust under it haha. Just my 2 cents.
 
So, basically, I cleaned with a solvent instead of water-based degreaser, I didn't treat with zinc phosphate or even just phosphoric acid, and I used spray paint. Not kicking myself as I just didn't know and am learning as I go, but now I know what to do and not to do. As for the powder coating, ideally that's what I should have done this time, or epoxy as some have advised. But POR-15 is pretty robust and hopefully with all the prep work it'll take and do the job. It's all underbody parts so if they don't look show quality that's ok.
 
Hey, automotive painter here. Your thoughts are correct as to why the rust returned. There's a reason it's called car cancer, unless rust has been completely removed or neutralized, it WILL come back. You should use a chemical called Ospho, which is a phosphoric acid, with another couple chemicals mixed in as well. But it comes in various sizes, and it's a green fluid that neutralizes rust and turns it into iron phosphate, which is inert, so it's safe to be painted over. The treated rust turns black, and I would suggest repeat applications if there's still some orange or red still visible in any pores or anything like that.

For how to fix this situation, I would suggest stripping down to bare metal if you haven't already. Next, use the Ospho to treat all the metal. I usually just use a microfiber, but make sure to wear gloves. It is an acid after all. After that, let the parts sit for up to 24 hours to dry completely. What I usually do once it's dry, is do another quicking sanding with 180g sandpaper to knock down any extra material or anything that may have stuck to the pieces. Then you'll want to use an etching primer, which also contains a small amount of phosphoric acid, to be your first coat of anything. The acid in etching primer not only helps prevent rust from forming again, but it also allows it to bond to the metal better, creating better adhesion. This type of adhesion is called chemical adhesion, as opposed to mechanical adhesion which would be sanding, media blasting, etc. Etching primer does NOT need 100% coverage to work. You'll want to apply it as a heavy mist coat. What I mean by that, is not completely solid where you can't see the metal underneath, but not so light that more metal is showing than is covered. Once the etch primer dries, you can then follow it up with another primer, usually a heavy body primer or a filler primer. Sand the dried primer, and paint like normal. Rust should never be an issue again.

As far as cleaning, what was used the first time probably didn't help the situation, but I doubt it caused more problems than the rust. For this go around, you'll want to use a grease and wax remover to clean the surface before ANY painting. You can get it by Rustoleum at Walmart or auto parts stores.

Is the goal to use rattle cans again, or do you have access to an air compressor and a spray gun? If you don't have access to anything other than rattle cans, most automotive paint stores should be able to put any color in a spray can, and it's meant to be used with a 2 part clearcoat, which will provide you better and longer lasting protection than the standard lacquer or acrylic paints that most rattle cans are. Some examples of an automotive supply store would be Auto Body Express or Finish Masters.
 
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Your suggestions are basically what I'm now doing, more or less. Get as close to bare metal as possible, water-based degreaser, phosphoric acid treatment and etching, better kind of primer and paint, etc. Except I'm mostly using POR-15 instead of paint, and intend to use KBS XTC for some exhaust components when I get to them later this summer.

Also, instead of just phosphoric acid, I'm using products that also have zinc in it, e.g. POR-15 Metal Prep. I assume this is ok, and perhaps even better. Although, it's kind of expensive compared to just phosphoric acid, so if you know of a cheaper alternative, that would be great.

I think my biggest mistake was using phosphoric acid-based products to remove rust and not ALSO treat whatever rust remained. I just rinsed it off, not realizing its dual purpose. Well, triple purpose, since it also etches the metal for better adhesion.

And, if I ever do this again, I'll probably get the parts media blasted as it's so laborious to get rust out manually, even with proper power tools, especially for those inevitable odd-shaped bends and curves and hard to access nooks and crannies, and for pitted rust, which takes so much time to remove properly and leaves the metal looking like the surface of the moon.

We're talking underbody parts like control arms, subframes, calipers, etc., so no need to go all-out with fancy paint, filler and sanding. When I do eventually tackle the exterior, I'll obviously have to adapt my methods as you do want to get things looking as close to perfect as possible. But I'm leaving that for last, working gradually from inside out.
 
No worries! Figured I'd provide a little more info for ya since you said you were learning. If you have any other questions, feel free to shoot me a message. I've got a rust bucket on the side of my house rn, and it's not the DSM LOL. 66 mustang that's rusted out.

But yeah, POR-15 is really all I'd trust for keeping rust out once it's been treated. I plan to hit the entire under carriage of mine before I move up north, cuz it's been a Florida car with very little rust. I'm sure there's cheaper products, but I wouldn't trust or recommend them like I would POR-15.

In the paint world, as a general rule of thumb, the more expensive the product is, the better it is. There aren't many instances you can go with a cheaper alternative and get the same result. So just keep that in mind. For prep chemicals like that, I'd only trust industry leaders.
 
Thanks, I like being corrected/instructed/guided. Only way to learn from your mistakes!

And, I assume that when you take on this derusting project, you'll media blast it? Grinders and rotary tools are way better than drills/drivers, but they're still really slow-going and beyond a few parts it really makes no sense to do it this way, I'm finding out. I've literally been doing little more than derusting all the parts for the past 6 weeks.

Since I work on the street everyone sees me working every day and they must think I'm crazy. Maybe I am?!? :p But, gas tank, tank shield, pipes, exhaust shield, pin brackets, subframe, diff, support, arms (upper, lower, trailing), sway bar, calipers, axles, axle shafts, flanges, nuts, bolts, brackets, etc. That's a lot of parts and a lot of surface area, much of it convoluted and hard to get to. Next time, blasting!

But it's almost over. Just a few more parts to derust, clean, treat and paint, and it's all going back on the car. I'm shooting to have it all done early next week.
 
If you're referring to the Rustang, no. Replacing entire body panels where I can, and sectioning what I can't replace. I've got collision repair experience, so it's pretty much the same as having to section a crushed up car, except I'm doing it cuz rust.

But as far as parts and stuff, yeah, media blasting is the best way. It'll get into any pitting a LOT better, allowing you to then protect that as well. Otherwise, get some sand paper from Walmart or something, and get to hand sanding. On metal like that, starting with 80g will be fine, then moving up to 180 and priming.
 
What I've done in the passed is clean every bit of rust I can off components, buy a few gallons of cement etching acid (muriatic acid) and soak the parts in the acid until they're free of rust. The acid will eat metal fyi! So don't leave it in long term, new acid will eat the rust rather quickly. I then rinse with distilled water. I have noticed the acid doesn't do any damage to paint tho, so don't feel the need to strip any good paint.
Then get wax and grease remover or the prep stuff from POR clean the parts thoroughly, and por15 them. If there is paint still on the parts anywhere sand it with 600 grit and reclean the part before painting.
That's how I do all my stuff.
 
I've started using muriatic acid but have found out its limitations.

First, as you wrote you can't leave parts in it too long or it'll eat the metal.

Second, you have to neutralize it with a base solution like baking soda and water, then rinse that off well.

Third, you need to clean the derusted metal and then treat it with phosphoric acid, to protect it from future rust. If possible you should then apply paint or something similar like POR-15 to further protect it.

Fourth, you don't want to use it on complex or critical parts as it could get into little crevices where the neutralizer might not reach and continue to eat away and oxidize the metal.

And fifth, the stuff is just nasty. I used a cupful outdoors a few weeks ago to derust some fasteners and my eyes burned and were very sensitive to sunlight and I felt kind of weak for several days after, just from the fumes. I was using the Klean Strip brand, which is around 15-20%, which I then watered down further 1:1, and it was still potent.

In the future I'll probably only use it for hardware I intend to reuse, that isn't structurally critical, like harness brackets or dust shield bolts. But mostly I've been avoiding using it when something else works just as well, like phosphoric acid, Evaporust or some abrasive or wire wheel. It's just too difficult to use and I reserve it for the nastiest rust.
 
I don't think rust reformer is made for high temperature applications. My guess is it lost adhesion from the heat and allowed moisture to enter the substrate.

If you used an acid to remove rust, make sure you neutralize it with a base and distilled water before top coating.
 
For the calipers I used their caliper paint, which is heat-rated. But I think the rust returned because I didn't properly neutralize it and treat the bare metal, and because the paint isn't very tough the calipers got scuffed and scraped and exposed the underlying metal to the elements. I'm finally starting to understand how not only to get rid of rust but prevent it from coming back again. I only knew the first part back then.
 
This was 2 years ago. I just stripped them down again and used POR-15 caliper paint. If it doesn't take then I'll powder coat them. I've also heard good things about epoxy paint but I'm not sure if it takes heat sell. Learning as I go. Everything I've painted so far is mechanical or structural and usually not visible, so it's mostly about rust treatment, not esthetics.
 
Been saying this since day 1 haha. Goes for ANY metal part you can remove. Other wise wire wheel it, use the por15 metal prep then por15 "paint".
What does powder coating cost, typically? Say, for a pair of single piston calipers. This is assuming I give it to them stripped to bare metal and they do the prep & coat.
 
What does powder coating cost, typically? Say, for a pair of single piston calipers. This is assuming I give it to them stripped to bare metal and they do the prep & coat.

It can honestly vary. When I was looking around for places in Cali I would just call the shop and ask for a quote. Once I got one kind of in the middle I would then look up their work and reviews and go from there. The place I ended up going with was so good on price, professionalism and quality that I am even going to SHIP most of my stuff to them from Denver area. Still have to find someone local to do the huge things like the subframes, but the shop in Cali said let me know who I am looking at and they will call them and vet them for me hahaha. So moral of all that is find a place that is both fair on price and treats you well and the quality will follow.
 
I guess it also depends on what your goals are and of course budget. For underbody and other rarely seen parts, looks are not unimportant but rust protection is tops and they don't have to look showroom or show car amazing. With my car being 30 years old and having spent so much time and money on it, it would probably be overkill for me.

For the body, of course, different considerations apply as you can see it. My hood looks terrible, like a dry lake bed, and eventually I'm going to have to do something about it. I assume that powder coating is used there, or more epoxy or something else?

Here's an image I just took of the subframe, diff & other connected parts after reinstalling them under the rear after derusting, treating & applying POR-15 & spray paint:

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To me this is good enough, if not quite show quality, especially if it keeps rust away. And yeah, I took apart and did this with the prop shaft as well. Basically everything rear of the t-case other than the bumper support and exhaust, which I'll get to eventually. The front I did already but parts may have to be revisited since I didn't treat them properly.
 
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