How to know if Ospho is Neutralized

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RHR

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Hi All,
This hood was media blasted, then sanded with 80g, then sprayed with Ospho and wiped with red scuff pad. Then it soaked at least 30 min. Then soaked with Ospho again for min 10 min, then rubbed down with water and red scuff pad. I did my best to keep it wet before and during rinsing with water and wiping with scuff pad and water. Then dried with air gun. Now I'm doing final sanding with 80 grit for epoxy. Based on what I've read, the small rust pits remaining are treated and should be fine under the epoxy. There was/is a dark gray coating on the metal that I've removed most of but some still remains.
How do I know if the Ospho is neutralized? What is that dark gray coating? Thanks all for any advice.
 

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Flash rust is OK. Stuff that happens in the moments after treating it with Ospho then rinsing. If you have any black pits of rust, you need to remove them or it will fail in that spot. Try going over those pits with a wire wheel brush then treating it with Ospho one more time. Soak it on there let it work, when ready to neutralize, re-wet with Ospho, keep it wet with Ospho for 10-15 minutes, taking care to not let it dry. After that time rinse well with water. Blow off.
 
Thanks Chris. Here's a close-up of an area. I previously sanded and then used end brushes to try and remove all the rust and then I used the Ospho. It appears the Ospho turned my Rust spots into Iron Phosphate which are are now the black dots. I thought as long as I neutralized the Ospho used to turn them black it was safe and good practice to epoxy over them.
Are you saying I need to remove all the black spots or it will fail?

Also still curious to know how you can validate you've neutralized it all.

Many thanks.
 

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If you keep it wet with product for 10 minutes then rinse you can be sure it's neutralized. As for the pic those black areas need to be clean/gone. It is not safe to epoxy over them. Wire wheel then ospho, or ospho and scrub the pits with a wire brush (hand) continuing to wet it and scrub it till it's gone. Then neutralize.
Me, I would wet it with Ospho, come back in 15 minutes, wet again and scrub, then wait some more time and do the same, scrub and wet again. When it looks clean neutralize.
 
RHR said:
It appears the Ospho turned my Rust spots into Iron Phosphate which are are now the black dots. I thought as long as I neutralized the Ospho used to turn them black it was safe and good practice to epoxy over them.
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It is not and it is not good practice.
 
I'm blown away that media blasting didn't remove all the rust, black dots and all. What media did you use to blast?

Also, I'm not understanding the need to sand with 80 grit over media blasted areas?
 
Thanks Chris. Got it. What's the best practice for the areas behind support panels in doors, fenders etc? I did the same Ospho treatment but can't get to those areas to remove the rust.

MJM. I had the blaster guy go real easy on the flat panels to avoid warping. Rust does not come out of those tiny pits easily so you have to be careful when blasting. I'm not sure what media he used but it was probably more for the quick removal of the majority of paint and rust. I like to get the area smooth with 80 grit so it is easier to clean with the W&G remover prior to epoxy.
 
Are you using soap in your washing stage?
After the rust is converted, you wet it one more time with Ospho and then wash it off using soapy water. The soap is what neutralizes the acid.
 
RHR said:
MJM. I had the blaster guy go real easy on the flat panels to avoid warping. Rust does not come out of those tiny pits easily so you have to be careful when blasting. I'm not sure what media he used but it was probably more for the quick removal of the majority of paint and rust. I like to get the area smooth with 80 grit so it is easier to clean with the W&G remover prior to epoxy.
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Understood. Your process definitely won't hurt anything that I'm aware of. Standard operating procedure when media blasting and using SPI epoxy is to blast, blow media dust off with compressed air, then spray epoxy. Wax & Grease Remover is not usually used after media blasting. The surface is already clean.

However, some will spray SPI 700 wax & grease remover drenching the panel after media blasting. Media blast panel, blow media dust off, drench panel with SPI 700 letting the 700 run off the panel carrying away any foreign material, blow dry. I'm not a fan of only drying SPI 700 with compressed air. I feel doing it that way could leave a film on the panel.

Most will not wipe a media blasted panel off after spraying wax & grease remove because the media blasted area will grab onto towels leaving lint.

Thanks for sharing your process. It interests me how others achieve tasks.
 
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'68 Coronet R/T said:
Are you using soap in your washing stage?
After the rust is converted, you wet it one more time with Ospho and then wash it off using soapy water. The soap is what neutralizes the acid.
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That's not really true although it doesn't hurt anything doing that. Neutralize is a misnomer of sorts. What you are doing when rinsing is removing the acid film. It can only be rinsed away when it is wet with ospho or other similar phosphoric acid treatment.
 
If you feel you have to use W&G remover on a media blasted panel, the easiest way is to soak the panel till it is dripping off. Then let it evaporate on it's own. You have to flood the panel for it to be effective. Like MJM said above many of us go straight to epoxy after blasting it and blowing it off. If you are using clean media and get the metal white it works fine not cleaning. I can understand though if it has been sitting or handled why you would want to clean it. If you don't want to go to the trouble of 80 grit then flooding it works to clean it. Just misting it on there and not wiping will not clean it though, important to flood the panel so it's running off. W&G remover works by lifting the contaminants from the surface, normally followed by wiping. By flooding it, this allows the lifted contaminants to run off the panel.
 
Chris_Hamilton said:
That's not really true although it doesn't hurt anything doing that. Neutralize is a misnomer of sorts. What you are doing when rinsing is removing the acid film. It can only be rinsed away when it is wet with ospho or other similar phosphoric acid treatment.
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I remember being told that a long time ago when I first started using it. Dawn Dish Soap has a PH of 8-10 so figuring Ospho is around a PH of 3, it would stand to reason that the soap would help neutralize the acid.
 
Dawn is very close to neutral, which is 7 on the ph scale, and clean undeluted water is about 7/neutral. Dawn is very good for what we use it, because its easy to rinse off without any residue. I like to use a stronger cleaner to rinse phosphoric acid off because it makes the metal less likely to flash rust, and I usually leave metal bare for long periods of time.

Zep Industrial Purple Cleaner is a good one, then rinse with clean water. Holdtight, and Tide detergent are also good, but only the powdered Tide, the liquid is not so high on the ph scale just like Dawn, but powdered Tide must only be used with Hot water to be sure all of it is desolved. These types of cleaners need to be sanded and cleaned with 700 before painting. Back in the 1960s hot rod magazines use to recommend cleaning machined engine blocks with Tide, and I think powered was the only one back then.

BTW, metal needs to be sanded after treating with Ospho, because it etches the metal by way of disolving it at varying speeds, which means it will disolve those sharp edges of the sand scratches quickly.
 
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