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Prep over old primer/rust

Doug Kimball

Subscriber
Last Subscription Date
12/10/2019
Working on a 12 hp Stover RH. It was sandblasted & primed approx. 15 years ago by P.O. then left out in the elements. Now a patchwork of old light gray primer & rust. In previous attempts in similar cases I've power wire brushed & re-primed then brush painted two coats only to have it peel after a couple of years. My question is how do I properly prep for brush paint to prevent peeling. Most pieces too large to put in my blast cabinet. Suggestions appreciated.IMG_1202(Medium).jpegIMG_1391 (Medium).jpeg
 

Talsie

Registered
Working on a 12 hp Stover RH. It was sandblasted & primed approx. 15 years ago by P.O. then left out in the elements. Now a patchwork of old light gray primer & rust. In previous attempts in similar cases I've power wire brushed & re-primed then brush painted two coats only to have it peel after a couple of years. My question is how do I properly prep for brush paint to prevent peeling. Most pieces too large to put in my blast cabinet. Suggestions appreciated.View attachment 382843View attachment 382844
If it were me i would sand blast it again and prime it with some good primer. That way YOU know it was done right.
 

Kevin O. Pulver

Email NOT Working
Age
54
Last Subscription Date
02/14/2020
I was also going to recommend the Ospho. It is phosphoric acid and looks like mint mouthwash and taste lemony/acidic if you get it on your hands.

I'm a little confused about the consistency of my results. I have used it where it'll turn rust absolutely black. I believe they call that iron phosphate? but other times it doesn't seem to turn the rest black the same way.

Really worth looking into this product.
 

Kevin O. Pulver

Email NOT Working
Age
54
Last Subscription Date
02/14/2020
The difference in colours is the result of the acid working on rust or on the steel. Good steel will not discolour but the rust will. It has done its job.
I understand what you're saying, and that makes sense. But the issue that I'm having is that the rust itself doesn't always turn black.
To be honest with you I even called the ospho Company to ask.
But the answer didn't seem to make any sense at the time. If it did I think I could have remembered what it was.
 

cobbadog

Registered
What did they say?
I think that you are safe to go to the next step and primer it up after a good wash down, dry and wipe over with a tack cloth. Get good coverage with your primer/undercoat allow it to dry properly as per the instructions then get you top coat ready to go. At all stages look carefully that you have totally covered everything you want painted and don't try to apply the paint too thick, build the layers up slowly and allow some "flash off" time between coats.
 

Kevin O. Pulver

Email NOT Working
Age
54
Last Subscription Date
02/14/2020
What did they say?
I think that you are safe to go to the next step and primer it up after a good wash down, dry and wipe over with a tack cloth. Get good coverage with your primer/undercoat allow it to dry properly as per the instructions then get you top coat ready to go. At all stages look carefully that you have totally covered everything you want painted and don't try to apply the paint too thick, build the layers up slowly and allow some "flash off" time between coats.
That's just it, I can't at all remember what they said. I thought the directions said that you didn't have to wash it off before painting? I need to go to their website and refresh my memory. At any rate when it turns the rust black, it's the most amazing thing!
My question was why it doesn't always seem to do that.
 

Doug Kimball

Subscriber
Last Subscription Date
12/10/2019
I copied this from Old Jalopy Journal:
"I noticed that it's still tacky for quite awhile after it's applied, but I noticed it starting to turn the rusty metal black pretty quickly...this was after 6 hours or so. I fully realize it says to give it 24 hours to cure, and if it rains (as its going to) or is humid (which it will be) that the reaction could cycle through AGAIN. I also noticed areas where it collected when I was brushing it on eventually ran down to whatever edge it could find and made this gummy mess.

I wanted to see how others use it, how they apply it, and how they handle the aftermath. I hear some folks saying you need to have a water hose and ScotchBrite pad to clean off the extra residue...some folks (and the package) say to just brush off the white powder and paint away. I want some real life stories of success and failure "


The above describes my experience with OSPHO. They say you can just paint over it but the residue is powdery and does not look fit to paint. If I have to apply it, wait 24 hrs., wash it off, dry it, then scoth-brite it to remove to residue I'll try something else. I apply OSPHO & primer/paint by brush.
 

cobbadog

Registered
The one I use here says to apply it and doesn't mind how, by brush or in my case I had a large area to cover so I filled up the garden weed sprayerd used that. I rinsed it out thoroughly after use. After 24 hours the rusty bits were black and the non rusty bits were not discoloured at all. Then I hosed the lot off and let it dry. Once dry I had no powdery surface so I sprayed the primer on over that.
I did do an experiment on the checker plate floor on Lorry the truck's tray. I just splashed any excess acid onto the tray and never washed it off. It did go milky and a little powdery but I left that and over time the surface rust has been sealed and the powdery look has gone. SO I can only guess that by hosing it gets rid of the powder.
 

RustyNumbat

Registered
Thats held up really good for 15 years outdoors... But you know the proper answer is the most annoying one - get it blasted again and start from scratch.

Some implements I put outdoors a few years back, the parts I treated with rush converted then painted with multiple layers of paint has held up far worse than the one piece I sandblasted and painted with a simple single coat of primer. Moral of the story - get the thing to bare-ish metal for best results!
 

Mike in NC

Moderator
Staff member
Age
46
Last Subscription Date
04/15/2020
After sanding or wire wheeling, use a self-etch or an epoxy primer. Then top coat it. Also, make sure that you clean the surfaces real good before applying primer or paint.
 

cobbadog

Registered
If it is a smooth surface I suggest that a light rub with some 240 grit to help 'key' the paint better, then the usual cleaning then primer and so on.
 

Mike in NC

Moderator
Staff member
Age
46
Last Subscription Date
04/15/2020
You do need some "sanding scratches" in the under layer. This promotes adhesion. They also make "adhesion promoters" that go under the top coat. This might be something else you am want to look in to.
 
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