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Discussion Starter · #1 ·
I have a 66 mustang stripped down to the frame with nothing but the rear differential on... I was planning on sandblasting the underbody, using rust prevention phosphoric acid called Ospho before applying a SPI epoxy primer that I have then later seam sealing it. So I have some questions:
1.)Does this all sound right and is this a good order to do this in?
2.)SPI epoxy primer acts as an undercoating correct?
3.)Do you have to sand down Ospho before apply the SPI epoxy primer?
4.)Epoxy primer go on before or after seam sealer? Thanks in advance
 

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I don’t think the ospho is required, epoxy straight to bare metal. Prime it shortly after blasting. I would top coat the epoxy with something. The seam sealing can be done before or after epoxy. Some will say they want epoxy on all bare metal, honestly it doesn’t matter. Go find some original sealer on the car and look under, I bet it’s pristine.

If you’re simply painting the undercarriage I would say seam seal before epoxy and immediately top coat it per mfg instructions. If you have metal work to do on floors pans etc then just epoxy seam sea after you do the metal work then top coat it.

I just give the advice I don’t follow it. My car has been sitting in the garage with bare metal for some time 😬 Gonna need to hit that with some 80 or blast again
 

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I blasted mine to bare metal,then seam sealed every gap it was reasonable to seal, then applied epoxy primer directly to the bare metal....then when over the top of the epoxy primer with acrylic enamel paint myself.
 

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You don't want to apply epoxy over ospho. The epoxy won't adhere well unless you neutralize it and if you're blasting you don't need to use ospho anyways. As far as using seam sealer it's personal preference when you apply it but I'm a fan of putting primer on then seam sealing then topcoat over it. Or if you're just priming put a coat on then seam seal then put the second coat of primer on.

I am a believer in if your sealer would ever come off you don't want bare metal underneath it that could rust so put a coating on then seam seal is the way to go.
 

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Barry @ SPI specifically states that his products are not compatible with Ospho or other PA products.

Here in New Orleans, flash rust happens in hours due to the high humidity, so Ospho was required, followed by Master Series Silver.

I love MSS, very easy to spray!
 

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Edit: I actually did put the primer down before seam sealer, seam sealer was between epoxy primer and the acrylic enamel. I won't be driving it unless the forecast is clear anyway, so I don't expect it to make much practical difference. I also won't be undercoating the car in any way...but I do plan on doing an undercarriage inspection and touch-up for things like rock chips on at least once-a-year basis.
 

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1. Blast followed by a quick D/A or scuff.
2. Treat with Picklex, then epoxy, if overly concerned about rust, or go straight to epoxy if not. If you use Picklex(or Ospho) use PPG DPLF primer which is compatible with phosphoric acid treatments.
3. Seam seal.
4. Top coat with a single stage urethane within the 7 day topcoat window of DPLF.
 

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The SPI epoxy (which I’ve used a lot of) is very fickle over acid, and the acid needs to be properly washed away first. I don’t treat anything with acid if I’m putting SPI epoxy directly over it.

instead what I do is put the Mastercoat Permanent Rust sealer (silver) directly over the acid and then spray the epoxy over the silver, and this is a truly bullet proof combo that will never rust on you.

I also would recommend using the Mastercoat metal prep over the Ospho. It contains a detergent and zinc in addition to the phosphoric acid, so it does a good job cleaning the blasted metal as well as providing additional rust protection from the zinc.
 
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What Lizer said is great advice. That will indeed provide a bulletproof combo. Also if for some reason you didn’t want to use Master Series silver, you could in the alternative apply Kirker Enduro Prime 2-part epoxy primer over the phosphoric acid. The Kirker is compatible with phosphoric acid. Also, you can simply use a brush to apply both the Master Series silver and the Kirker epoxy. They’re both thick so a brush is the best way to apply both.
 
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