Son of a *&(#$@!!! Freckles of rust showing through paint on my firewall....

Not a response to your current problem, but the thread was headed in this direction...
I use both solvent based cleaners and water based cleaners. One or the other won't get everything. Clean really well before you start removing paint so you are not grinding oil, grease into the bare metal. after everything is stripped off, use the solvent and waterbased cleaners again. Zinc phosphate etches the surface to give it some tooth, which is why por-15 requires it. Keep the zinc phosphate wet on the surface, working it with a red skotchbite until the surface turns grey. Any surface being por-15'd can be done right over the phosphate coating, but any surface being epoxy primed must be zitterbugged down to bare metal (it still leaves the phosphate in the pores and scratches, cracks etc.)just before priming. If you try to epoxy prime over the zinc phosphate coating you will have an adhesion problem. Be particularly careful to get it out of inside corners, where the lack of adhesion will cause the primer to pull away from the metal as it shrinks.
I wouldn't do a car without the metal prep(zinc phosphate)...like everything else you just have to learn to use it correctly. Ask me how I know.
Edit: the coating I was refering to is the grey coating. There is also a yellow residue which needs to be removed. The yellow residue comes of easily with water and skotchbite pad. The grey coating has to be sanded off.

 

Man, I'm no painter and certainly not an expert on the subject bit in my little experience if there is any rust that is going to show up from underneath the paint, it's going to take a while to do so and also it will show up as little pimples. My guess, like others above, is that it is either small metal fragments that got blown into the paint when you shot it or it is grinding dust on the surface. As others mentioned, try rubbing it out on a spot and see what happens.

 

The standard process is not complicated, just takes a minor amount of discipline and a maybe a just a touch of creative thinking at times.

No matter how you strip, the metal prep phase should never be eliminated. It etches, or what some say, "pickles", the steel. It stinks in enclosed areas, you need rubber gloves, you have to develop a bit of an eye for what you're looking for. What to look for? 1st, consider water on an oily surface. You've seen how it spreads away or looks like fisheyes on other things. Until the metal's properly etched it'll do that. If the solution doesn't stop it then you need to employ an abrasive pad. I like red scotchbrites and simply throw em away afterward. So, now we're etched and it's time to rinse it off. Water on bare steel is clearly counter-intuitive, right? There's that flash rust issue, but w/out water to neutralize nothing will hold to the metal. The trick is to dry it up as quickly as possible, as well as get it neutralized. One method of quick drying is very hot water. The heat helps evaporate the water quicker but the use of compressed air is still mandatory and immediate. As it drys you'll see a blue and gold haze form on the metal. That tells you it's ready, willing and able for a coating. There's areas where that neutralizing can be of some concern, like where it might be hard to dry up or even rinse well. If it's like that I use baking soda. Yes, good ol Arm n Hammer baking soda mixed in with the water. You'll see it foam up doing it's job, but again you want to flush that away with hot water and dry it up quickly.

Even though flash oxidation is a concern there's actually more time than you think to get it safely coated. Many say within minutes, but unless you're doing it on a foggy midwest morning that's not really the case. If you're not in a spray booth or some other controlled environment, get a simple house fan to move the air around and keep things dry until you get your primer mixed up. Again, this won't work in a Londonesque fog, but in most shop conditions it makes a nice ally to your task.

How well does this work? I could show you some cars done in the late 80s to early 90s that still look like "last year" with no issues, still able to take top scores in judging. And isn't that what we all want? Longevity? The ability to enjoy all of this work for years to come? There's only 1 thing worse than doing this type of work. That's doing it over!! Follow those simple steps and you shouldn't have to.

Now, as an aside, you might just want to check and see if you got some grindings in the top surface as mentioned. Unless you're a totally solo act, or keep an eye on the project at all times, it's extremely possible that you're victim of that. I'm forever surprised at the lack of logic I see around shops, even a**** some of the "good guys", but that's another topic all together. Good luck and let us know what you find out.

 

This topic goes back and forth. Do not use metal prep if your DTM primer says not to. There is no such thing as a "standard process". Different brands require different processes. To say metal prep is required no matter what...is wrong.

 

I bet its grinding dust left to rust on top of the surface. Like what was said above, try to rub out an area with compound or a claybar before you get carried away repainting. Unless it has been many years outside, and you rattle canned it, I doubt it's rust coming through the paint. Even dust from wearring in new brake pads can give you the same effect. Good luck.

 

X3 - Done that too...

 

He's in Calif. do you have etch primer there? Was it water based paint? Proper procedure?

 

The entire process involved grinding down the paint with a wire wheel and a purple disc. I then welded plugs into the holes to smooth the firewall. As I progressed I would clean with wax and grease remover until the cloth came up clean and when the wax and grease remover was thoroughly dry I shot the area I just finished with self etching primer. I would do segments at a time due to the size of the project, but would always re-scuff the fresh metal surfaces when beginning work the following day and would be sure to feather into / scuff into the edges of the surrounding areas that had already been primed previously, then reshoot with self etching primer. When the entire firewall was done I scuffed the entire thing and shot with building primer, sanded, smoothed, added building primer, etc. The final coat of building primer was shot the day prior to paint. On the day of paint I wiped down the entire area with wax and grease remover until the only thing that came up was a slight gray hue from the primer, waited for it to dry completely, and then shot 3 coats of color, allowing 15 minutes of flash between coats and making the final coat heavier and shiny. The final result was very nice and even, but I will admit that on the downturn of the firewall (the surface that sits at a 45 degree angle facing the floor) I did get a few dry spots. I thought my prep was thorough, but maybe I missed something.

I am hoping that this is in fact grinding dust that settled and rusted on the surface. I did have some grinding going on nearby and there has been opportunity these last few months for raw metal to rust.

 

Suppose I got a fresh piece of pickled steel, or acid etched steel. How would I remove the etching process?

Here's where many get confused. There's conversion coatings and there's prep solutions. Metal prep LEAVES NOTHING ON THE SURFACE. To say it does...is wrong.

Phosphate soultions form crystals on the metal to enhance adhesion. There are indeed some primers and coatings that will not adhere to those crystals, or may actually remove them from the metal taking the topcoat with it. Metal prep is a liquid way of giving the metal a "tooth" to promote adhesion, nothing more. Phosphates were good for old tech finishes like lacquer and lacquer based primers that tend to flash and dry quickly. They would help give those finishes a chemical bond as well as a mechanical bond. Metal prep is STRICTLY mechanical bond.

Now if something has changed in these products over the years I didn't get the memo, but until I see that it leaves more than well etched steel I'ma keep doing it.

 

A neighbor of mine and his dad had a similar experince 2 weeks after repainting his small wooden outboard motor boat.
They realized shortly after the pimples appeared that they final sanded it with STEEL WOOL.
That stuff rusts almost overnight when water gets spashed on it.

 

"IF" the back side was panted the front painted don't you need air and moister to get rust

 

The backside is the floor pan / toe kick area of the inside of the cab...it's not painted. Still not sure how rust could form that quickly.