Ospho, filler, and water

4

494Sparky

New Member
Absolutely new to this so I can hear the eye rolls already. Never painted so that process kept getting pushed back. I started my metalwork and as I was new to that it was frustrating. I broke up my metalwork by doing bodywork over completed areas of metalwork. Generally used a first coat of short strand fiberglass over welds and finished with filler. Years later I am close to finishing metalwork but now I have a heavy layer of surface rust on rest of vehicle. My plan was to Ospho entire vehicle and neutralize/wash when ready to epoxy. My question is what effect will Ospho and water have on my finished filler? I also was reading some other posts and am now wondering what was left on my metal after chemical stripping. Description says it will be neutralized, washed and a rust inhibitor applied so metalwork can be done. Opinions/suggestions welcome. I assume fiberglass would be fine but filler?

Would definitely do things differently now, but this is what I have to work with.
 
A picture would be helpful. I try to avoid acid treatments as they can lead to other problems that I don't need. Will the rust come off with 80 grit on a DA ? That would be best, or some type of mild blasting media. The water won't hurt the filler so long as it gets 100% dry before any primer goes over it. I have run into problems with filler I thought I got dried but didn't get it completely dried. The sun and time are the best tools for drying completely. Anything you can do to keep the filler from getting wet will also eliminate future problems.
 
Can you post some pics of what you are dealing with?

Hmm, RenewAP types faster than me. LOL

Your situation is why the first thing that goes onto bare metal is epoxy primer.
 
20230625_192846.jpg
 
Exteriors could probably be sanded but door interiors? How heavy is too heavy to sand rust? Also under dash etc. that would be tough to get. I have some areas that have been ground on pretty extensively and may be getting thin also??
 
That rust is surface rust and will sand off fairly readily. Unfortunately, it is probably under the filler as well so all the filler would need to come off also. That is definitely not a gamble you want to take. So I think your best course of action at this point is to remove the filler unfortunately, completely sand the entire panel down, and then spray with epoxy. As a rule of thumb whenever you have bare metal, the first thing you should strive for is to get it in 2 to 3 coats of epoxy.
 
Media blasting has become my starting point on cars with a lot of rust in hard to reach places.
Here you can see where I started on the firewall area. I will go over it until it is free of any rust.
Media Blasting Progress 2.JPG


Next is epoxy primer, two coats:
Firewall in Epoxy.JPG


I planned on replacing the floor pans so didn't blast them.
You can do your filler work over the epoxy primer and this eliminates the chance of any rust forming underneath. Also the car can sit like this for years (inside out of direct sunlight) and then just be lightly scuffed and a fresh coat of epoxy applied and your ready to continue to primer and paint.

Passenger Side.JPG
 
Lizer said:
That rust is surface rust and will sand off fairly readily. Unfortunately, it is probably under the filler as well so all the filler would need to come off also. That is definitely not a gamble you want to take. So I think your best course of action at this point is to remove the filler unfortunately, completely sand the entire panel down, and then spray with epoxy. As a rule of thumb whenever you have bare metal, the first thing you should strive for is to get it in 2 to 3 coats of epoxy.
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I needed to hear this. I will grind it all off before proceeding. Any other options inside the doors? Ospho seems like the best bet here to get everything clean and wash it out before epoxy.
 
494Sparky said:
I needed to hear this. I will grind it all off before proceeding. Any other options inside the doors? Ospho seems like the best bet here to get everything clean and wash it out before epoxy.
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Sorry about your luck. A knotted wire wheel on an angle grinder makes fast work of stripping filler. Then go over clean bare metal with 80 grit on DA again.

Inside the doors what you can do is take paper towels saturated with your acid and lay them on the rusty metal. Put some saran wrap over them to keep them from drying out. After sitting for a couple hours they will start to dissolve a lot of the rust on the inside of the doors and you'll start seeing clean metal. But you need to keep them wet.

I won't use epoxy over acid, it's just a risk I don't want to take and it's a pain to wash the acid off. For tricky areas like this inside doors, etc, where it's been treated with acid, I go directly over it with Mastercoat silver, which is an aluminum pigmented moisture cured urethane. It doesn't have adhesion issues with the acid and 2 coats will keep it from re-rusting. If you want to brush epoxy over it for color or additional robustness, you still can but it needs to be applied the following day. Otherwise, I prefer to blast whatever I can so I can start with a completely clean substrate.

Also resist the urge to encapsulate rust. Rust inherently contains water and oxygen at a molecular level and encapsulating rust will trap these, which can actually exacerbate rusting (or until the water or oxygen is completely reacted and the rust synthesis can no longer proceed).
 
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