Underbody Surface Rust

train42

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Aug 31, 2022
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Hello. New guy here. I have the opportunity to acquire an 89 Mustang LX 5.0 Convertible that has been owned by various family members for the last 25+ years. The car is basically all stock and in good condition with around 120k miles. There isn't any visible rust on the exterior, or the shock towers, but there is some surface rust underneath the car. The floor pans look ok. The car hasn't seen winter for the last 15 years, but it was driven through a few winters around 20 years ago. My question is: will the surface rust that is present now continue to get worse and worse and eventually cause structural issues, or is this not a big concern? Is there anything I can do now to minimize the rust damage from spreading? Here are a few pics underneath the rear of the car:

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limp

wrap a little cheese around it and its a done
Oct 4, 2020
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You can wire wheel all of the surface rust on the floor pan and then paint it... As long as you find no surprises..
Some items, like the rear end housing would be easier to do if you removed it to clean and paint it.....
 

General karthief

wonder how much it would cost to ship you a pair
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I figure this is not your daily driver so this is what I would do, I would pull engine/trans, rear end, then as suggested by limp, wire wheel the rust spots, this will give you the opportunity to check, replace brake/fuel lines that are suspect, and looking at that fuel filter you may need to, yes this means dropping the fuel tank, working your way forward inspecting shock towers, and frame rails, using something like Por15 to seal it up as you go. Cleaning, sealing up the bottom of this car will help preserve it.
Oh and consider adding subframe connectors while under there.
JMO
 
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evintho

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Nov 12, 2003
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I see you're from Ohio. Judging by the pics, that almost looks as clean as a California car! 120k on a 33 year old Mustang is virtually low mileage. The second pic however, gives me pause. The fuel lines/filter and brake lines are about what I'd expect from an Ohio car. Replacement is in your future. If it were me, I'd follow the General's plan exactly as stated!
 

limp

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In your first picture, just left of center is a yellow stripe and on the right of it looks like gasket flaking, or flaking rust?? What is that....
 

Gs1987GT

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That's about what my 87 looked like underneath before I sanded and wire wheeled the undercarriage and the brake and fuel lines. Personally I would not replace lines unless they are structurally deficient. Some light surface rust on the outside isn't a big deal. You could go look at a 5yr old car and they'd look like that (steel lines). I didnt remove the rear to paint it, but you can as general suggested.

I started from the front and worked my way back until I had everything prepped, then I painted using rustoleum semi gloss black. I did install MM full length subframe connectors first though.

I did sand and inspect my lines real good though and would have replaced them if any were questionable, but all of mine were fine.
 
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rednotch

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Depends how far you want to go, I would avoid taking wire wheels to the sheetmetal, it makes a a mess , and would use a rust remover like krud kutter/ rustolem rust remover or even vinegar soaked rags to deal with the surface rust first. then clean and prep it for a fresh epoxy sealer to blend into the factory coating..

the suspension and axle stuff Knock the heavy rust off with a wire wheel while its off the car then neutralize the rust, clean and prep for any decent paint, I would buy a girdle vs wasting time on the 8.8 cover though.

I've done a couple cars stripping them down to the shell on the underside, one using por15 that didn't hold up and started to bleed threw since it doesn't convert rust even using there metal prep, but I had the supplies for free from the Toyota frame recalls back then. .. Rest I used expoy that looked almost factory just missing the over spay and held up much better. The rustolem/Krud kutter rust disolver is the same product and is probably the best thing I've used for rust removal as far as speed and getting down to clean metal.. Vinegar is cheap and it works but its much slower and leaves a coating you need to scrub off before coating, either way it can be a ton of time in prep work.... One things for sure I will never ever buy an under coated car again, removing that stuff was a nightmare and it did a crap job at stopping rust.
 

limp

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Depends how far you want to go, I would avoid taking wire wheels to the sheetmetal, it makes a a mess , and would use a rust remover like krud kutter/ rustolem rust remover or even vinegar soaked rags to deal with the surface rust first. then clean and prep it for a fresh epoxy sealer to blend into the factory coating..

the suspension and axle stuff Knock the heavy rust off with a wire wheel while its off the car then neutralize the rust, clean and prep for any decent paint, I would buy a girdle vs wasting time on the 8.8 cover though.

I've done a couple cars stripping them down to the shell on the underside, one using por15 that didn't hold up and started to bleed threw since it doesn't convert rust even using there metal prep, but I had the supplies for free from the Toyota frame recalls back then. .. Rest I used expoy that looked almost factory just missing the over spay and held up much better. The rustolem/Krud kutter rust disolver is the same product and is probably the best thing I've used for rust removal as far as speed and getting down to clean metal.. Vinegar is cheap and it works but its much slower and leaves a coating you need to scrub off before coating, either way it can be a ton of time in prep work.... One things for sure I will never ever buy an under coated car again, removing that stuff was a nightmare and it did a crap job at stopping rust.
I bought two gallons of Muriatic Acid ( its used for swimming pools) at depot for $11.... I would not have believed how well it literally removes all the rust if I had not seen it for my self...... I bought it for my lower front control arm after spending two days wire wheeling one of them.......
I now use it for all sorts of small parts...... It is water soluble...... Wear safety glasses and a mask around it.....
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rednotch

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Well yeah you can't beat that stuff for stripping stuff but it can be a pita to store and dispose of, you don't want to keep that in your garage lol the fumes will eat up lots of coatings and flash rust any bare metal near it. Really have to make sure its sealed air tight if you don't neutralize it when your done..... I knew a girl since childhood who still bares the scare of getting it on her face when she was a kid, got splashed by it, burned her face and hands pretty badly,

The rustolem stuff is no where near as strong but its not going to burn ya if you use it on rags to keep the floor pans wet it also won't eat most coatings Muriatic Acid cant be beat on stuff you can dunk in a container and don't care if it gets stripped, but its not deffenitly something I would not want to be using on my back under a car lol.
 

nickyb

WAIT,you now have a pair?
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looking at his car i am blessed to have a rust free/ crust free southwest car, it was sold new in Cali. and has been in Nevada for most of its 29 years.
 
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limp

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The rustolem stuff is no where near as strong but its not going to burn ya if you use it on rags to keep the floor pans wet it also won't eat most coatings Muriatic Acid cant be beat on stuff you can dunk in a container and don't care if it gets stripped, but its not deffenitly something I would not want to be using on my back under a car lol.
Agreed.... I will be wire wheeling mine after I get the front end back together and up on my other set of Race Ramps...
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90sickfox

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That muratic acid wreaks havoc on metal. I used to service a pool company fleet. They bought new vans every 3 years because the acid would eat the floor of the vans. Nothing stopped it. I wouldn't use it either. Too worried about if it gets up between two panels and starts to eat them.

Rust converter works very well. I used it on an old 1988 E150 van I had. Sprayed the frame with it. The thing turned black and stayed like that for years. Not sure how it would work with spots that have paint around them. I also didn't paint over it.

Rustoleum has worked good for me on bare metal stuff. Doesn't work well over painted surfaces without a lot of prep. Learned that on one of my old engine bays. Looked good for a few months. Them the stuff started coming off. Flaked off the underside of the hood too.

Naval jelly works well but takes time and layers of coats. It was my " go to " for a long time but I think the formula changed. Seems like it got weaker.

I'll try the Krud Kutter rust desolver next.
 
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rednotch

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That muratic acid wreaks havoc on metal. I used to service a pool company fleet. They bought new vans every 3 years because the acid would eat the floor of the vans. Nothing stopped it. I wouldn't use it either. Too worried about if it gets up between two panels and starts to eat them.

Rust converter works very well. I used it on an old 1988 E150 van I had. Sprayed the frame with it. The thing turned black and stayed like that for years. Not sure how it would work with spots that have paint around them. I also didn't paint over it.

Rustoleum has worked good for me on bare metal stuff. Doesn't work well over painted surfaces without a lot of prep. Learned that on one of my old engine bays. Looked good for a few months. Them the stuff started coming off. Flaked off the underside of the hood too.

Naval jelly works well but takes time and layers of coats. It was my " go to " for a long time but I think the formula changed. Seems like it got weaker.

I'll try the Krud Kutter rust desolver next.


there both phosphoric acid around 30-40% unless they changed the formula again.... Liquid not the spray gel..... no where near as strong as muratic so it won't eat the base metal., it converts iron oxide into iron phosphate and leaves a coating that wont rust for awhile like most do, it neutralizes the rust, but you still need to prep it before coating it with a sealer like paint or epoxy if you want to protect it for good ... I would treat the rust first because just taking a wire wheel to it spreads the iron oxide everywhere and creates more rust and down right sucks to grind lol....,


Basicly you convert the rust, then wire brush/ grind / sand / spot blast the area to clean metal to prep for a sealer then paint., the acid takes care of the pitted areas from creaping back and lifting the coating but your still using a wire brush or spot blasting to get to clean metal in the pits after treating it, besides that only other correct way is to cut it out and weld new metal.... It works great a light rust like shown on those floor pans though where it hasn't pitted heavily yet... Epoxy, some high build and a dusting of base coat to blend it in and it looks and holds up like factory. Rust converters are just a phospate layer, they don't really seal anything, so there's no magic spray over rust product that will really last. Its alot of work either way.
 
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Mustang5L5

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That underside would be typical of a northeast car. I'd be more worried about the towers. That surface rust can be cleaned up well, but it's the towers/rails that will rot from the inside and not show it until it's too late.
 
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