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Phosphoric Acid

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rwalton1961

Member
I've got some pitting in sheetmetal on a tractor I'm getting ready to paint. Do you brush the acid on...or use a squirt bottle?? AS I understand, you let dry, then wash off with water?? What keeps it from rusting, if you use water on bare metal?? Thanks
 
Actually the phosphoric acid is intended to etch the metal so the primer can adhere better. If it were me I would rinse it off after you use it and then quickly dry the metal. In order to stop the rust you need to get some epoxy primer on it as soon as possible.
 
I have done both, but prefer the spray bottle. Keep it wet for about 10-15 minutes. That is usually enough time to work. Then rinse good. While always good to get primer on quickly, I've left it for days in my garage before painting. Granted I don't have a humidity issue (but don't live in the desert either).
 
I let it dry and sit with the dry acid on it until I am ready to prime. I then use a squirt bottle with diluted phosphoric acid, about four parts water to one part acid. I rewet it and wipe it dry with paper towels. Usually do that the night before and then prime it the next day. Right before prime it I scuff it down with scotch brite pads and wipe it down with wax a grease remover. I did this on an antique pickup and holding strong now for 4 years. Used Omni epoxy primer. Using the diluted acid for rinsing helps keep flash rust from occurring.
 
No, it won't hurt the old paint but you will still need to scuff sand the old paint regardless of whether you put paint or primer over it.
 
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