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Advice on refinishing doors
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70bus
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PostPosted: Tue Dec 06, 2022 4:53 pm    Post subject: Advice on refinishing doors Reply with quote

Hello; I'm trying to refurbish some split window bus doors, but obviously it's better to paint them beforehand. So I tested some grafitti remover on the top layer of paint - which was working well - and got to this point:


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I assume the tiger stripes are something liquid that ran down the door and made the overspray more vulnerable to removal. I also assume the bare steel and bondo starting to appear means I won't be getting a salvageable OG finish! Too bad; I liked the area around the handle...

To my actual question: did my local paint stop steer me wrong? I mentioned that I was intending to remove overcoats where possible, wash any bare metal w/ Ospho, and then paint, either to blend with what OG paint there is, or redo whole area if necessary. The shop sold me some rattlecans of L87 in PPG 2-part and an epoxy primer as well. Reading the forums, I see mentions that epoxy does not like Ospho! I suppose I can save those cans for areas I strip to bare metal if that's what needs to be done, but I did mention the Ospho to paint people and they never threw up a warning about it... only caveat given was that the PPG would only stick to epoxy primer.

So should I sand this door bare where there's no original paint, use the PPG, and feather it in where it meets OG? Use PPG over Ospho and cross fingers? Or get cans of something else and go over the Ospho-coated metal with it?
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Craig K
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viiking
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PostPosted: Wed Dec 07, 2022 8:10 pm    Post subject: Re: advice on refinishing doors Reply with quote

The issue with Ospho is that people do not wash off the residue. It usually states on most phosphoric acid based rust converters that you can't let it dry out. The phosphoric acid converts the rusty iron oxide to iron phosphate which is an excellent base for painting. Unrusted metal or paint will not react so the phosphoric acid dries out meaning it is a potential for reaction to any subsequent coating.

IF you have treated any areas and haven't done it correctly, I would reapply the Ospho, keeping it wet for the period of time you need to convert the rust. Then immediately wash the area with copious quantities of running water and wash with a rag to ensure any crystallised Ospho is removed. Ordinarily this is enough, but if you are concerned you can give it a good wash with some water/bicarbonate of soda mix which will neutralise any residual acid, then repeat the fresh water wash.Then dry with a rag, heat gun and then paint immediately with the epoxy primer. Of course you can go the whole extra mile and wash the surfaces with acetone/wax and greaser remover first.

If you haven't treated it with Ospho, clean and prepare the surface with sand paper, wash with water, dry as above then proceed to paint. Even if there is a little rust left, the epoxy paint will seal the surface from the air and stop it from continuing. That's why they call it a sealer. Remove the water or air from the steel and the rust process does not proceed.
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70bus
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PostPosted: Fri Dec 09, 2022 11:26 am    Post subject: Re: advice on refinishing doors Reply with quote

Ah; so it's the unbonded crystallized phosphate the paint dislikes. This was really helpful - thank you.

A bit of a topic swerve, but will the rpoxy primer stick to other primers? Body shop used panels from a few different suppliers, so I have green and various shades of grey primer on panels. If the PPG epoxy doesn't stick to anything but bare metal I'll have to go another route for those areas.
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Craig K
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viiking
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PostPosted: Fri Dec 09, 2022 2:11 pm    Post subject: Re: advice on refinishing doors Reply with quote

I would remove all the primer off any of the panels. You never know what they use and how good the adhesion is. Most suppliers would use cheap primer just for storage and transport purposes. Unless you know the paint is compatible with what you’re overcoating with, remove it.

Your paint will only be as strong as the weakest primer so why take the risk?

If you use paint stripper to remove paint then remember again to wash the surface with copious quantities of water-preferably very hot to remove any residue of the stripper followed by a solvent. Follow the directions carefully depending on whether you use a methylene chloride type or the more environmentally safe types.
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1966 Lancia Flavia Pininfarina Coupe - in the waiting room
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esde
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PostPosted: Fri Dec 09, 2022 7:06 pm    Post subject: Re: advice on refinishing doors Reply with quote

70bus wrote:
Ah; so it's the unbonded crystallized phosphate the paint dislikes. This was really helpful - thank you.

A bit of a topic swerve, but will the rpoxy primer stick to other primers? Body shop used panels from a few different suppliers, so I have green and various shades of grey primer on panels. If the PPG epoxy doesn't stick to anything but bare metal I'll have to go another route for those areas.

I've had SPI epoxy hold tight to the green primer that Klassic Fab uses, but it hates the black primer that comes on Dansk and Klokkerholm stuff. I use some lacquer thinner and remove that junk.
And as said, sand the unreacted Ospho back off. If it's black you can leave it, but the unreacted chalky stuff is NFG to paint over.
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viiking
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PostPosted: Fri Dec 09, 2022 7:31 pm    Post subject: Re: advice on refinishing doors Reply with quote

FYI .I specifically asked Klassic Fab about their green primer and was advised by them to remove it before painting.
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70bus
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PostPosted: Fri Dec 09, 2022 9:39 pm    Post subject: Re: advice on refinishing doors Reply with quote

Quote:
I use some lacquer thinner and remove that junk.

Well, you say that...

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There's more, but that gives a feeling; basically, 90% of the metal below the wheel arch tops is new. No way I can get all the primer. My strategy now is to epoxy/PPG 2-stage rattlecan all the parts NOT having the OG paint like doors, nose, etc. The rest will have to get some sort of overall primer that should stick and then single-stage stuff and maybe a clearcoat. I am still learning about paint beyond Krylon... but if Krylon's where I end up, that's ok. I don't have the $$ or time to showroom this bus, so I'm just trying to preserve what's there for the next idio - uh, lucky owner 10 years from now.

I'll eventually have more pics on a 'my bus' post but it's time I haven't been able to make yet. When up, it may lead to more suggestions. Asides from walking away and leaving it in the desert when that was an option!

But I appreciate all the advice; it helps me tease out a sane way to approach this w/ the resources/time/money I have...
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Craig K
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"If Von Dutch was alive, he'd walk in there with a pistol and shoot these people."
-Robert Williams
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esde
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PostPosted: Sun Dec 11, 2022 8:44 am    Post subject: Re: advice on refinishing doors Reply with quote

You're probably going to be fine. My comment about removing the primer from the Dansk/ Klokker parts is because whatever black primer they use is super cheap and wipes right off with thinner. Nobody is going to tell you the primer they use is ok to spray over, cause then if it fails they're liable. But my experience is that it's the underside of the bus and good surface prep is more important than taking everything down to bare steel. If it's the topside of a car that will get filler and finish paint then yes, take it to bare steel, IMO.
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