Phosphoric acid

[Screwdriver]

Going to rake a flying guess and suggest once the acidic reaction has completed, all of the acid has transformed into water during the process. Bit like a lead acid battery, once it's "flat" the acid has turned too watery.

As for water being able to persist in a tiny crevice, I think that is entirely plausible too. You'd probably have to really heat the shell up to drive out all the moisture.

All of which is pure guesswork. Only replying here because I am bored.

 

[Onoff]

Would something like dry bicarb powder or dye penetrant stuff "bring the acid out"?

 

[Torqu-E-Cycles]

Problem is the 2 skins of metal are so tightly sandwiched I can't get anything in between them, I'm surprised water would even make its way in, my only thought is maybe soak in wd40 or cavity wax for a week then panel wipe to death to get a base for paint to stick?

 

[Screwdriver]

Water or any other liquid will wick into a tight gap. Makes me wonder if superglue might help. Since it is activated by water it might use up all the moisture to form a plastic coating inside the seam. Just thinking out loud here, you'd probably need to experiment!

 

[Onoff]

Put the car in a bag of rice for a week.

 

[Gareth J]

GCSE chemistry coming back to me; Acid + Alkali= Salt + Water.

Steel isn't a source of OH- ions so shouldn't directly produce Water with the H+ ions of the acid (though some more complex chemistry that's beyond me might!) Bicarb will though, so maybe that's not a great idea.

That said, there will be lots of water in there anyway, the "acid" is mostly water. And I suspect that a mostly dry seam, with some acid/spent acid in it may have some osmotic potential too - drawing in atmospheric moisture.

Really hard to get all the water out of a tight seam, short of smothering the whole shell in old duvets and putting a heater inside, don't have any suggestions!

 

[Dcal]

Problem is the 2 skins of metal are so tightly sandwiched I can't get anything in between them, I'm surprised water would even make its way in, my only thought is maybe soak in wd40 or cavity wax for a week then panel wipe to death to get a base for paint to stick?

This was what I was getting at in my earlier posts (19 and 27)

If the seams are dry, any liquid applied to the surface will get drawn into them by capillary action.
I think getting paint or wax into the seams is the best option, but you probably don't have that option now.

However, I wouldn't worry about it too much.
The water / acid / base or whatever is in there, needs oxygen to react.
If it's sealed the reaction will stop fairly quickly when the oxygen gets used up, as long as you prevent more oxygen entering the seams and the epoxy should prevent that happening.

I'd let it sit for a while and see what happens then sand off the discoloured epoxy and touch up with more epoxy applied by brush and see if it happens again.

 

[MetalMonkey]

I'm having an issue on the mini roof, after using the acid as we've discussed, washing off with clean water and completely drying then buzzing off with 120 and epoxy priming I'm getting brown rust stains creeping out of the seams! I only painted it 48 hours ago and it's in the dry.

Surely if there was some acid hiding in the seam crevis it wouldn't cause rust as thats why we are using it on the first place?!

Out of the entire roof there's 3 spots which are tiny, the size of a match head but they're clearly coming from between the 2 sandwiched seems ☹

If you have been washing it off with water, that is probably where the issue has come from. I think your problems originate from initially applying the concentrated acid.
A properly diluted mixture of phosphoric left to dry should from a protective surface with no washing. Blow out any seams with an airline.

Once dry, I find it a good idea to abrade the areas I can get to so that you get a good key for paint, you may get white and powdery residue on clean steel, but should come with scotchbright and some prep fluid. Rust should turn black.

 

[Torqu-E-Cycles]

I have left it alone for about 5 days and checked it tonight, out of the entire roof and gutters there's only 3 little brown dots about 1mm dia which is exactly as it was 5 days ago, so whatever was in there hasn't got any worse thankfully!
Its impossible to get to the other side of the seam as there's a stiffner in the way, my only hope is to spend a while warming up a thin tube and forming a shape that will allow me to jet cavity wax in, heavily, messy, but hopefully it'll seal it up from the air.

I'm not too worried as this mini won't go out in the rain, it'll live inside a fairly dry garage and hey, if I have to populate the back seat with moisture trap boxes or even a small dehumidifier im not that worried! If it blows rust out of the seams in a few years time it'll have a new roof skin fitted.


Lesson learnt about using neat acid!!