Unibody rust on project... thoughts?

W

wca_tim

Member
first, I've searched and read here and many other places in recent months and I've been using epoxy primer (as well as other products) in a number of things with really pleasing results overall. It's what I'm planning to use on the underside of my current project - a 4wd project for weekending on and off road, mountains, beaches (salt!), etc...

the underside has some scattered surface rust (especially along seam areas ..grrr) as well as rust a few places (but not very many) that go in to "frame" rails. There was only one spot that was structurally weak (below the battery). I cut it out and replaced it. Overall I'm doing a LOT of welding on this - plating, gusseting and reinforcing the majority of the unibody structure, removing and adding brackets, etc.... It's not intended to be a work of art, nor is it going to be, but it will be plenty strong enough for what I have in mind. Once I get the majority of the metal work finished, I want to stave off rust expansion for absolutely as long as I can. (I've been working on it a couple months when I have time, and will probably be at it some weeks before "finished") I'm leaning strongly toward...

1. using da with 60 grit, wire wheel and needle scaler (only thing that'll get in to a lot of the corners, etc...) to remove as much of the rust spots and compromised paint that I reasonably can and to rough up the new metal metal surfaces. (don't care about surface imperfections..)
2. use a shop vac and / or compressed to clean out anything in the frame rails / unibody where I have access
3. use pressure washer with clean any heavily oily areas with soap and water
4. drench all of the surfaces with ospho using a garden sprayer and let it sit over night (being somewhat careful not to get much if any into the cavities in the unibody - may temporarily plug the drain / vent holes)
5. pressure wash it
6. spray it down again with ospho and soap, then pressure wash, flush well with water.
7. After it's dry, give the whole mess a couple wet coats of epoxy primer
8. once that's good and dry, do all of the cavities as well as the surfaces with some home brew cavity wax (beeswax, lanolin, rust inhibitors in a mineral spirits base - a variation on a mix that seems to be working really well on some other things so far, and it smells nice too :) ), and retreat / coat with that ever few years as needed...

am I missing anything? or is there something I can do that will work better for this sort of an application?

Thanks!
 
Is media blasting out of the question? Then you don't have to worry about acid residue inside of seams.
 
crashtech said:
Is media blasting out of the question? Then you don't have to worry about acid residue inside of seams.
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I had that thought - both the acid contamination and media blasting. Not really completely out of the question, but I'm not sure the mess it will make is worth it in this case... the engine and a number of other things will still be in the vehicle and I'm a little concerned about getting sand in places I don't want it. That I'm not exactly thrilled at the idea of doing the underside laying on my back... how difficult / important is it to be certain that all of the acid is gone out of the seams? Does thoroughly rinsing, power washing then rinsing again before letting it dry leave much of a chance of leaving a lot of acid in the seams?

I realize that there's no easy way to do anything that'll keep the rust from gradually eating away at this thing... I'm hoping that the "cavity wax" / undercoating will make a big difference - some of the old school surf fishermen I've met have OLD vehicles that still aren't rusted out too bad even after living years on the beach in the salt. They just coat the crap out of everything underneath with some mixture of linseed oil, lanolin, beezwax, parafin wax, motor oil kerosene, etc, etc... every year or so. The other alternative would be to not even paint the upper parts, just the new metal I weld in, knock the heavy rust off and put a heavy coat of "cavity wax" on everything... and keep an eye on it to see what happens.
 
From what I've seen, acid getting into seams is bad even if it's rinsed, because it seems to activate the metal and you'll have rust weeping from every seam. I'd be interested in hearing what some of the other guys have to say about it!
 
I've been reading more on ospho... from what I gather (and I should know some of this), alkyd paints are fine over ospho that's completely dry - without rinsing or neutralizing. I understand that good quality alkyd paints combined with an aliphatic isocyanate hardener can approach the durability of good clear coat, and wouldn't be affected by residual acid nearly to the extent epoxy will... I'd rather use spi epoxy based on my experience with it's great durability, but if I can't do anything to help with the rust that won't be a problem, may be the sort of situation where old school would be best. I habe to admit though, I've used por-15 metal prep and a quick rinse with spi more than once and have had zero problems with adhesion. one case was on an aluminum outdrive, another on a set of rims... anyhow, I've got a week or so before I have to make a final decision, so any experience that might help would be greatly appreciated.
 
crashtech said:
From what I've seen, acid getting into seams is bad even if it's rinsed, because it seems to activate the metal and you'll have rust weeping from every seam. I'd be interested in hearing what some of the other guys have to say about it!
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This goes against what I have experienced with weak phosphoric acid solutions such as ospho. Did you experience this, or see results from someone else's use of acid? I have left phosphoric acid on bare metal for years without a problem, it doesn't promote rust, it protects against rust, unlike the strong acids that can rust metal with just their fumes. Strong acids seem to open the pores of the metal to the point that no matter what you do, that metal will always be prone to rusting.
I have seen muriatic acid recommended on a couple of forums as a very inexpensive product to use for removing rust, but it is one of the worse acids to use and will do just as you mentioned, and I'm thinking this may be what you have seen.

The only problem I have noticed with phosphoric acid is with paint adhesion if it is not properly removed before epoxy. If the doors are to stay on the car, then I would just use cavity wax. If the doors are off the car, then I turn them upside down when I rinse the acid off.
 
Do you mean bare metal exposed to the elements and repeated wetting, or metal that's kept dry? There is a phosphate coating left behind by the acid that in theory does protect the metal, but in practice, it hasn't seemed to work out too well for us if the residue can't be rinsed, and a coating can't be subsequently applied in a reliable way.

BUT, I'm not an expert on the weathering of treated bare metal, that's for sure, so I will leave it to others to supply better informed advice.
 
crashtech said:
Do you mean bare metal exposed to the elements and repeated wetting, or metal that's kept dry?
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Definitely metal that has been in the shop the whole time. It sounds like you are talking a conversion coating, and I don't use or trust them to cover up rust and protect the metal, I just use acid to remove rust.

To get 99% of the residue to rinse off, I use a maroon scotch pad and/or a SS wire brush to scrub the thoroughly acid wet metal good just before rinsing. What little residue that is left will easily sand off. This also seems to remove the zinc phosphate, so it will be more prone to flash rust.
 
Just a reminder to never store muriatic acid in your garage! I used some to remove rust and crud on inner door parts that wont be painted. I mixed it up in a 5 gallon bucket in the driveway and set the remaining jug on the garage floor behind my Olds. Knowing that muriatic acid will fume and cause rust, I keep the 5 gallon bucket sealed outside. Then I forgot about the remaining jug. My Olds rear quarters have been bare metal for the last 5 years. Dry climate, no rust. About a month after the muriatic acid, I lifted the cover, and the lower 12" of the quarter panels are rusted over!

Not sure what I will have to do to remove that rust. But I'm not likely to get back working on that car for at least 2-3 years.
 
That rust is very aggressive, I would blast it if possible. You can remove it with phosphoric acid but it might take a lot of SS wire brushing with the acid, just depends on how long it has been exposed to the fumes. I wouldn't use muriatic acid for anything around a car, you should check the areas you cleaned with muriatic also.
 
I used it to clean up window regulators, hood hinges and latch. Cleaned up great, rinsed immediately and scrubbed with Dawn and rinsed. Hood hinges will get powdercoat. Rest will be left alone.
 
All good thoughts, thanks!

The vehicle I'm working on is a full size dodge conversion van - there's a LOT or real estate under there to blast all of it or even just all the seams. I'm not so worried about making it look pretty. In fact I'm intentionally leaving the body and paint kind of beat up and faded (perhaps it'll be just a little less likely that someone will try to break into it looking for my toys...). I'm mainly looking to stave off the rust as long as possible without getting into the kind of time it takes to do it "right". It feels a kind of odd to intentionally half ass something like this, but as long as it stays structurally sound for the foreseeable future, I'm probably ok with it... I'm honestly looking to the "cavity wax" / oil to help slow any rust that's started up in the seams and unibody "frame" rails... That and covering areas where things will splash in easily while leaving places to drain when it does...

It would probably be too much to ask of Barry to ask what the linkages are in the spi epoxy resin formulation so I could do some homework and see what kinds of reactive / redox rust preventative(s) might be compatible... :) but that's something I could look into if I had an idea what the hardener functionality and pKa is... I haven't done any epoxy chemistry for a while, but I'm guessing that acid kills it by pronating the anime hardener... ;-)
 
The process might be more effective if creating a passivation layer is attempted instead of just an acid etch. We used to use PPG DX520 after acid to produce a grey layer of zinc phosphate.
 
good thought... (btw. 520's got LEAD in it! - not a lot, but it doesn't take a lot...). I was thinking that the por15 metal prep stuff I've used quite a bit might be the ticket... it's dailute (ie 8-10%) phosphoric acid with about 5% zinc chloride... My experience is that if I do final prep on aluminum, mild steel or stainless using that, quick rinse with water, let it dry and then shoot epoxy over it, the epoxy bonds great (clear urethane sticks pretty well too... ;-) ). if I saturate everything from inside the rails and above with penetrating cavity wax after the epoxy is good and dry, I'm thinking that is probably the best bet... rust needs oxygen and water to progress, limiting both from both sides seems the best bet..
 
POR-15 is shite. Methinks you are waaaay overthinking this. If you epoxy it properly what's in the seams will stay there. Should not be an issue. Media blast, then 2 to 3 coats of epoxy making sure you have very good coverage. That's your safest play. Use a acid product proceed at your own risk.
 
http://www.spiuserforum.com/index.php?threads/why-you-never-use-por-15-ever.4204/

Look over my post on that crap. If you still choose to use anything from that company don't come back here and ask why, it's all on you.

The pics were reposted after they were lost during the site change SPI was going through and couldn't place them using the EDIT feature so I just reposted them in the order they first appeared. Have a look and decide for yourself.
 
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Thanks everyone very much for the experience and insights! I'm going with the general approach of removing all of the rust I possibly can with needle scaler, flap discs, spot blasting, wire wheel / grinder, etc... then phosphoric acid, rinse, soap and water... followed by plenty of time for the seams to dry out. after it's all done and cured, I'll saturate the interior of everything I can with a good wicking "cavity wax" see how it holds up with time... That's the tough part on this kind of stuff for someone like me - by the time I know what might work the best over time on a big / time consuming project, it's too late.

re: por-15 above: I don't / didn't use por15 rust paint - my own experience with it some years ago wasn't great and I do pay attention to what (you) folks tell me... Their metal prep product is available for purchase locally as is their marine clean (pretty decent waterborne oil and grease remover). Both are decent products in my experience. The metal prep product is just phosphoric acid with a few percent of zinc chloride added... I can't find ospho anywhere locally, but do have a source for pure orthophosphoric acid.

Note also that the Glisten clear is a good product in my experience. - polished aluminum Imco powerflow marine exhaust manifolds, shot with a couple heavy coats of glisten clear after metal prep. still look pretty damn good after 8-10 years riding in a hot, humid, boat bilge often with a little brackish water. They've been misted with oil, sprayed with power steering fluid and had both gas and antifreeze spilled on them and had hoses rubbing against them with little evidence of abrasion. It didn't hold up so well on a mercury racing lower that was polished - I can't think of anything that might though. I also shot a little over silver flake that rides outside, you have to look to see it, but it's yellowed some in the sun over time (somewhat expected with the high amine content in those products). The back "Hardnose" so far is the toughest paint I've tried on boat outdrives / lower units

Just my experience - a company may have a number of products, because one doesn't perform well, doesn't necessarily mean that another won't be a good fit for my application - and vice versa...

Shooting leaf springs and front axle, spring perches, etc... with spi epoxy this afternoon so I can assemble in a couple days - they're looking good to me :)

Thanks all again for sharing your information, experience and insights and to Barry for excellent products at attractive price points!
 
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