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AndrewS85
Samba Member
Joined: May 13, 2013
Posts: 42
Location: Upland CA
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 Posted: Tue Mar 19, 2024 9:08 pm Post subject: Roof pitting & phosphoric acid, what's next |
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I had a fair bit of rust on the roof of my bus which was sandblasted, only problem is that the blaster couldn't get the center areas that had heavy pitting for fear of warping the metal. So I took the ginder with a flap disk and a wire wheel to get what I could. Then I used klean strip concrete and metal prep (phosphoric acid) to treat the deep parts of the pits. Now I have decently clean areas with black spots of converted rust in the pits. Next step will be to neutralize and clean it tomorrow with water as the manufacturer recommends.
So hears my question, what goes next? Filler to fill the pits, DTM primer to seal it off then filler?
I plan to call the manufacturer of the filler and the primer tomorrow to see if either will claim their product is compatible to go next, just looking for advice.
Last edited by AndrewS85 on Tue Mar 19, 2024 10:22 pm; edited 1 time in total |
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74 Thing
Samba Member
Joined: September 02, 2004
Posts: 7395
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| Posted: Tue Mar 19, 2024 9:25 pm Post subject: Re: Roof pitting & phosphoric acid, what's next |
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https://www.youtube.com/watch?v=6HOonWprmdw
Lots of info in that clip.
Have to determine if you want a mechanical bond or chemical bond.
Then determine what kind of primer you are going to use and whether to use filler under primer or over.
Lots of decisions. |
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AndrewS85
Samba Member
Joined: May 13, 2013
Posts: 42
Location: Upland CA
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| Posted: Tue Mar 19, 2024 10:28 pm Post subject: Re: Roof pitting & phosphoric acid, what's next |
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| That's a good video, but it's about surface rust. My question is specifically about the areas of pitting that have converted rust |
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busman78
Samba Member
Joined: August 17, 2004
Posts: 4525
Location: Oklahoma City, OK
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| Posted: Wed Mar 20, 2024 4:07 am Post subject: Re: Roof pitting & phosphoric acid, what's next |
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| I would use a wire wheel, stay away for sanding, metal is only so thick and wire wheel will clean out most pits, then neutralize. Use DuraBond for the first coat over bare prepped metal, this will fill the pits, sand with 60 gtit, now have a good base for skim coat of filler, sand an prime. |
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Evil_Fiz
Samba Member
Joined: May 06, 2011
Posts: 1049
Location: Jacksonville, FL
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| Posted: Sat Mar 23, 2024 6:53 am Post subject: Re: Roof pitting & phosphoric acid, what's next |
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| busman78 wrote: |
| I would use a wire wheel, stay away from sanding, metal is only so thick and wire wheel will clean out most pits... |
This is good advice. I will add that you probably want to hit the pitted areas again with the wire wheel. The black substance is converted rust but it may not be all of it. Get it all out before continuing.
| busman78 wrote: |
| ...then neutralize..... |
You will need to reactivate the acid to neutralize it.
- Reapply the acid to the previously treated area
- keep it wet for 10-15 minutes (reapply acid as needed to keep the area wet)
- Scrub the pitted and surrounding areas with a red or brown Scotch-Brite pad to dislodge any loose rust and contaminants
- Before rinsing, liberally wet the area with an alkaline degreaser such as ZEP purple or Dawn Ultra diluted in water
- Rinse the area with plenty of running water to neutralize and remove the acid. Scrub/agitate the surface with a rag/towel as you rinse it. Epoxy and acid film do not mix, so make sure the acid is completely removed
Finally, Protect the metal and perform the desired bodywork corrections
- DA or hand sand the area with 80# sandpaper to provide tooth and a mechanical bond for the epoxy
- Clean the area with a solvent-based wax & grease remover, liberally wetting the area with the W&GR and then wiping clean with a dry, lint-free cloth (do not use microfiber towels, they can cause adhesion issues). A ZEP Chemical resistant spray bottle (Lowes/Home Depot) is ideal for applying the W&GR
- When you THINK the area is clean, use a water-based (mostly alcohol and acetone) W&GR to remove any organic or remaining contaminants (skin oil, etc.)
- Apply 2-3 coats of high-quality epoxy. You can brush or spray it on. You can also dab extra epoxy in the affected areas to aid in filling in the pits. Epoxy is very flexible and will move with the metal better than filler will
- Allow the epoxy to fully dry for ~48 Hrs.
- Apply the first coat of filler after 48 Hrs and within the recoat window for the epoxy. Most filler manufacturers recommend applying filler directly onto bare metal. This recommendation is borne out of a CYA abundance caution. It has been shown that filler will chemically bond better to epoxy than mechanically to metal.
- Apply filler, block sand, rinse-repeat, until satisfied
- Apply 2 more coats of epoxy to seal in the filler
- Sand, prime, color, and clear as you normally would
Epoxy will provide a moisture and oxygen barrier over the bare metal and filler. It will also provide a superior chemical bond with the filler. Once you are satisfied with the filler work, top-coat it with another 2 coats of epoxy to seal out moisture.
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Emil
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“…It's not just about what's interesting. It's also about what's helpful, and it's helpful even if it helps just one other guy working on a Ghia.”
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See my build on TheSamba at:
The K_R_A_K_E_N_N : a 70 Ghia Convertible reinterpreted |
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Braukuche
Samba Member
Joined: September 03, 2004
Posts: 11005
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| Posted: Sun Apr 14, 2024 9:25 am Post subject: Re: Roof pitting & phosphoric acid, what's next |
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You have to get rid of all those black pits or it will come back. It’s rust.
I’ve used Naval Jelly covered with plastic. I let it wet overnight, wipe off, hit with wire brush on grinder. Any left, I repeat until the bottom of the pits are shiny metal. Then seal or do body work over.
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Go Reds! Smash state!
Retirement is here!
1956 Ghia
1959 SO-23 Westfalia
1960 double cab
1960 Baja Bug
1963 stretched double cab
1962 Golde sunroof Ghia
1963 356 B coupe
1963 Notchback
1967 21 window less rusty now |
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