Ospho after sandblasting ?

JohnMartin

JohnMartin

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I sandblasted a couple engine tin pieces from an old vw. They came out really nice. I figured I needed to etch the metal so I sprayed them with Ospho. Did I need to do that ? Should I have just hit it with 80 grit or will the ospho do the trick ?
 
JohnMartin said:
I sandblasted a couple engine tin pieces from an old vw. They came out really nice. I figured I needed to etch the metal so I sprayed them with Ospho. Did I need to do that ? Should I have just hit it with 80 grit or will the ospho do the trick ?
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If you Ospho'ed it after blasting you will need to neutralize the Ospho before applying epoxy. Simply re-wet with Ospho, keep it wet with product for 10 minutes or so then rinse with water. If you don't do that epoxy won't adhere.
In the future if you blast you do not need to do anything else. You can go straight to epoxy after blasting, provided you use fresh media.
 
It takes heat or time for phosphoric acid to etch the metal. It etches by eating away or dissolving metal at an uneven rate, so that is what it did when you put it on blasted metal, giving the metal a smoothing effect because of all the sharp edges that can be dessolved quickly.

To test this, just dip a piece of well blasted metal in a container of acid and watch it fizz, as it quickly dissolves the sharp edges/thin metal. I always put a sanding profile on the metal after using acid, and that also helps get rid of the acid residue.
 
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I have one more question about this. Here’s a before Ospho and after Ospho comparison. When I sandblasted these pieces I got that nice clean grey look. I thought that was the metal. But the Ospho ate threw a layer or two after the sandblast to a darker looking metal. Can anyone tell me what happened there ? Did I not sandblast good enough? Should I have gotten down to the darker metal or is the light grey acceptable?
 

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Chemistry is what happened there! You've reacted the phosphoric acid with the metal to form a layer of iron phosphate on the metal.

Congrats scientist. Your blasting was fine. If you want those parts to remain as bare metal, coat them with ATF and wipe the excess off, they'll stay nice for a long time.
 
Lizer said:
Chemistry is what hap pened there! You've reacted the phosphoric acid with the metal to form a layer of iron phosphate on the metal.

Congrats scientist. Your blasting was fine. If you want those parts to remain as bare metal, coat them with ATF and wipe the excess off, they'll stay nice for a long time.
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I have wondered if it's possible to coat phosphated/Parkerized parts with a crosslinking oil like linseed oil, and if that would function well as a protective, or just be a gummy mess.
 
JohnMartin said:
View attachment 25157I have one more question about this. Here’s a before Ospho and after Ospho comparison. When I sandblasted these pieces I got that nice clean grey look. I thought that was the metal. But the Ospho ate threw a layer or two after the sandblast to a darker looking metal. Can anyone tell me what happened there ? Did I not sandblast good enough? Should I have gotten down to the darker metal or is the light grey acceptable?
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I have to say that doesn't look like blasted metal. Is it possible that you got your pictures mixed up, and actually posted one with the parts painted gray?

At any rate it looks like the black spots on the Ospho treated parts might be areas not cleaned good enough, those areas would have to be scratched to see if they turn brown, to be sure. Unclean areas will really show up when using acid after wire brushing or blasted metal that looks like it might be good enough. Properly cleaned badly rusted areas can also look dark after phosphoric acid treating.

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I like to use a red Scotch bright pad on the acid wet metal just before rinsing to get much better results.
 
chevman said:
I have to say that doesn't look like blasted metal. Is it possible that you got your pictures mixed up, and actually posted one with the parts painted gray?
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Hey chevman, no those grey looking parts are the way they looked after blasting. Not sure if vw has some kind of coating around the metal. It’s from 1970. Here’s a pic of before blasting. I just blasted and Osphoed I never painted them yet.
 

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Looks like you still have a layer of paint or undercoating on those parts.
What are you using for a sandblaster?
 
I asked around , apparently Volkswagen coated those with zinc chromate to help prevent rust back in 1970.
I just using the cheap harbor freight setup with 41 gallons of air.
 
JohnMartin said:
I asked around , apparently Volkswagen coated those with zinc chromate to help prevent rust back in 1970.
I just using the cheap harbor freight setup with 41 gallons of air.
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I blasted my 1970 Type 1 tins with crushed glass and they look like yours. Mine had the factory finish and very little rust. Either we're both at bare metal or we're both not ;) In fact, my blasted chassis, front beam, and all suspension parts look the same as the tins. I think you're fine.

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Emil
 
JohnMartin said:
I asked around , apparently Volkswagen coated those with zinc chromate to help prevent rust back in 1970.
I just using the cheap harbor freight setup with 41 gallons of air.
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Acid will remove zinc plating. That’s how I strip zinc from nuts or bolts if I have to weld them. Between the blasting and acid bath you should have removed all of it. Zinc is a sacrificial coating so it degrades over the years.
 
Lizer said:
Acid will remove zinc plating. That’s how I strip zinc from nuts or bolts if I have to weld them. Between the blasting and acid bath you should have removed all of it. Zinc is a sacrificial coating so it degrades over the years.
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There certainly wouldn't be any left in the rust pits, and the reason it even started rusting is because the zinc coating was used up. I still think you mixed up your pictures, and accidently posted the wrong one.
 
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