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topic 7423

Paint Not Sticking to Ospho

A discussion started in 2001 but continuing through 2018


Ospho Rust Converter

Q. I built a piece of equipment using regular mild steel plate and standard structural angles and square tubing. I wire brushed all of the powder rust and spray applied a coat of Ospho. After 24 hours it was completely dry to the touch and I applied a coat of Rust-oleum #7435. 48 hours later the paint could be easily peeled off with the back of a fingernail!

Question one: What did I do wrong?

Question two: What do I do now?

Giles Van Duyne
- Groveland, Florida



1. How did you prepare the surface prior to the ospho?

2. Did you de-grease the surface?

3. What time of day did you paint ?

4. How was the paint applied ?

5. Did you notice anything unusual about the application (crawling paint, fisheyes, sheeting, etc.)?

Steve Cook
- St.Petersburg, Florida


A. The whole trick to painting on metal with an oil based paint is to be sure that the surface is Clean with a capital C. This not only means grease and dirt but also water. Or more correctly, water humidity which will get on metal in a damp and cool day.
Heat the metal up to about 85 degrees so that moisture does not want to collect on the surface.
That way the metal will be bone dry.

Marvin Alvarez
- Bridgeport, New Jersey



- OLYMPIA, Washington


A. Although the Rust-o-leum product you applied may have been dried to the touch which most coatings are, it may have not been fully cured. Most paints, even epoxies, have a 30 day full cure time. You can scratch off most coatings even after a 4-5 day time frame, after 30 days you should not be able too.

Dan Burnett
- Gainesville, Georgia

April 4, 2011

!! PLEASE STOP SPRAYING OSPHO! If you aerosol this stuff, use the proper protection. The MSDS requires a certain respirator if the stuff is airborne. You do not want to breathe phosphoric acid in an airborne state especially if you're in an enclosed area.

Ken Burge
- Houston Texas


Q. Hi all.

I've just come across this site while researching rust converting products. I've recently purchased a '93 Explorer with quite a bit of rust. So far, I've bought an aerosol spray called "Rust Converter [linked by editor to product info at Amazon]" and a bottle of Ospho. I originally thought the two accomplished the same end result. But reading several posts here leads me to believe this is not the case. Is one of these products superior to the other? Would I achieve better results if I were to use both (the Ospho and then the Rust Converter)? I had intended to spray the frame with the Rust Converter and then use an undercoating. As for the body rust, I planned to use the Rust converter, then prime and paint. Now that I've found the Ospho, I'm not sure how to proceed. Any help would be greatly appreciated.

Jennifer Bennett
Hobbyist - Bunnell, Florida

A. Hi Jennifer. Rust converter and Ospho are pretty much the same thing, so I say use just one or the other.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Q. I bought the Ospho product. What is the best way to apply it? What type of prep do I need to do to the door beforehand ( One of the responses to another question said clean it well. WITH WHAT?

It is a sectioned door with a white powder coating(?) paint and textured like orange peel. The surface has a light coating of rust all over. Door is structurally sound, but looks bad. After treating with Ospho, what type of paint can I use over it?

Debora Cross
hobbyist - Cape Canaveral, Florida

Radio Flyer Flew
from Abe Books


(2007) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have an old radio flyer wagon from my husband's childhood that I would like to refinish. I'm not sure where to start? Could you please help me out with the best steps for this project? Thank you so much!


Jackie Riesberg
- LaVista, Nebraska

May 17, 2009

Q. I have used a combined rust converter and primer on tiny rust stone chips on a vehicle. Alas I didn't wipe away the excess and now have much larger brown stained patches where the product dried which seems very difficult to remove. Does anyone have any ideas apart from the obvious option of sanding the stained surface away please ?

Frances Grindlay
- Melbourne, Victoria, Australia

sidebar (2007)

Q. I'm restoring a 1923 Aermotor windmill. The rustoleum aluminum is too bright. I would like the dull galvanized aged look. Any way to tone down the aluminum? I've tried computer matching at Home Depot with an old blade. Auto paint can match close but is for spraying and I'm brushing/rolling. Thanks, Dick

Dick Strough
hobbyist - Fortville, Indiana

September 24, 2009

A. To the windmill restorer. You might try "Cold Galvanizing" which is a spray on galvanizing paint. It does have a duller appearance. It can be purchased just about anywhere that cans of spray paint are available. Galvanized metal can be aged, or etched, by wiping with vinegar which is a mild acid. I would give the Cold Galv thirty days to cure first. I've not tried this on Cold Galv but have on galvanized metal many times.

Bob Johnson
- Ocala Florida

November 19, 2012

A. If you apply a thick coat of ospho on metal surfaces it creates a waxy film. The primer failed to adhere to the waxy film. In my experience, after applying ospho, we scrubbed the surface with mineral spirits to remove the excess ospho. This allowed the primer to bond. The primer has not failed after twenty years. These are civil war era cannons that are exposed to the elements year round.

Mike Hosti
Retired - Tybee Island, Georgia

Ospho pretreatment of metal roof before painting

February 25, 2017

Q. I have a 10,000 sq ft metal building that is 30 years old. The galvanize steel metal pan roof is getting some rust scale on it. I bought 2 steel bristle push brooms to knock off the loose rust scale and will pressure wash the brush the loose rust off with tap water before applying the ospho when the water dries. I plan on applying the ospho with cotton mops at the center top crest of the roof into each roof pan starting at the top and working the ospho downward. Reading other texts I'll apply on a windy day re. fumes. Question: does this procedure sound ok? Should I pressure wash the roof after the Ospho dries? I will be applying a new long life polymer roof coating afterward. The roofing supply house recommended the Ospho pretreatment for the first phase of this job

philip van winkle
- orlando, florida, usa

February 2017

A. Hi Philip. I believe that Ospho tells you to rinse it before painting to remove any white residue. But it sounds to me that as long as you don't have the pressure so high that you strip off the Ospho conversion coating, a pressure washer ought to do a good job at it.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

July 5, 2017

Q. I have a cast iron wood stove, that I'm trying to refinish. The stove is expected to be in a fairly humid environment, and there's no way around it. There's minimal investment in the stove, it's a hand me down. So far I've purchased some steel cup brushes, a steel brush, a phosphoric acid based rust converter similar to ospho, and a can of 248903 rustoleum High Heat black.

Pictures of what the stove looks like currently:

7423-1a   7423-1b   7423-1c   7423-1d   7423-1e   7423-1f  

So far I've run the cup brush on a 1500 rpm drill (the fastest that I have) and I've managed to remove rust and paint from the stove. I want to know do I need to completely remove the paint from the stove to apply the ospho and the rustoleum?

It is difficult to remove all the paint with the drill and steel brush. If it is suggested I remove all paint, Can anyone suggest a quick and affordable way to remove paint without an excessive expenditure of tools?

Finally any recommendations on the final step of applying the phosphoric treatment?

Colin Williams
- Shoreline, Washington

Steel boat - applying a 2 part epoxy primer to an OSPHO treated surface

April 5, 2018

Q. I'm treating and painting my steel boat.
I've needle gunned & wire brushed the scale and rust followed by treating with the OSPHO (a phosphoric acid treatment) to the cleaned surface.
I'm advised that the Ospho treated surface won't take a 2 part epoxy paint primer.

Can I rinse and clean the treated steel with water followed by a thorough wipe-down with denatured alcohol to rid the steel of the Phosphoric acid contaminant and then apply the 2 part epoxy primer ?

Will this work ? or Any other suggestions please.

Thank You.

geoff orr
engineering - Ft Lauderdale, Florida.

July 17, 2018

Q. Hi,

I have a steel boat that was built in 1950. The hull is in exceptional condition for her age; however, I do have a slight issue that I want to sort in the best way possible. Below the waterline the boat has been replated at some point --because of this the steel hasn't enjoyed the same level of protection as the rest of the steel on the boat. Under the engine (which is extremely hard to access) I have what looks like coal tar epoxy that is wearing thin or totally gone leaving a thin rust layer or a degraded surface. My plan was to wire brush as best I can and then phosphoric acid the steel. Then, after washing and drying the area I intended on using a coal tar epoxy. Is this the right thing to do or is there an even better method? Any help on this would be really appreciated.

Paul Langham

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