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Classic/Antique Car Repair/Using Phosphoric Acid on Metal


Good morning,

I am getting ready to use some phosphoric acid and "Rust Bullet" on various metal panels on my 1967 Ford F100 Pickup.  The phosphoric acid is called "Prep and Etch."

Here is my question: Should I wash off the acid (with water) after 30 minutes or so?  Or should I leave the phosphoric on the metal overnite and then wipe it off with a damp rag?

Thank you for your help.


The manufacturer of these products recommends washing the product off after 15 or 20 minutes. I have tried it both ways and find that the makers directions do the better job. Then if you are going to paint over it use the directions for the paint and the primer as well. I have never used Rust Bullet and their web sire recommends calling them for application information. But my base rule of thumb is to follow the makers instructions to the letter.

Classic/Antique Car Repair

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Brad Sears


All automotive including antique and collectible. However if the car has been modified I can only answer in general terms and maybe get you pointed in the right direction.


Automotive tech instructor. Syndicated auto columnist 1970's though the early 1990's. Syndicated auto radio talk show, Ask Brad About cars, CBS Radio 70's through 90's TV Show "Last Chance Garage" 1980's PBS-TV syndicated. Auto instructor for the following companies: Fram Autolyte Holly Carter AMF Ford Motor University Of Conn Blue Hills Technical School Sugar River Technical Center Grew up in a family garage in Needham Mass and turned wrenches from the age of 14.

Manchester Union Leader, Nashua Telegraph, Motor Service Magazine, Yankee Magazine, Popular Mechanics (Saturday Mechanic early 80's), Los Angeles Times, New York Times, and lots more.

More than I care to remember. Basically Franklin Technical Institute in Boston, Northeastern University, Fitchburg State Teachers College, Tufts University, and a lot of factory schools along the way.

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Moto Award winner. And much more.

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