Painting over Phosphoric Acid

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CJ13

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Our keel is badly rusted & needs Blasting back to bare steel & repainting.
The contractor is proposing the following:
1.Slurry Blast,
2. Pressure Wash to remove contaminants
3. Apply Phosphoric Acid prior to Epoxy
4. Apply 5 Coats Epoxy
5. Anti Foul

I've been told that Phosporic Acid has poor adhesion properties and isn't a good substrate for Epoxy. Does anyone know if this is true?

Thanks for any advice.

John
 
My understanding of epoxy coating is that it should be applied immediately to freshly blasted iron. pressure washing doesn't seem to fit in with this.
Blasting standards are available for this type of work.
Phosphoric acid converts iron oxide, ie rust, to a more stable form. Seems your contractor is expecting to not clean the keel fully, not a good foundation for epoxy IMO.
 
JFowler said:
Our keel is badly rusted & needs Blasting back to bare steel & repainting.
The contractor is proposing the following:
1.Slurry Blast,
2. Pressure Wash to remove contaminants
3. Apply Phosphoric Acid prior to Epoxy
4. Apply 5 Coats Epoxy
5. Anti Foul

I've been told that Phosporic Acid has poor adhesion properties and isn't a good substrate for Epoxy. Does anyone know if this is true?

Thanks for any advice.

John
Click to expand...

You're missing primer I think too :)
 
Repeated Pressure Washing was recommended by the paint manufucturer, to remove contaminants from the Slurry Blast & voids in the Keel Casting. The Keel would then need any surface rust mechanically removing before painting. I think the Phosporic Acid avoids this step, but am concerned about bonding.

John
 
JFowler said:
Repeated Pressure Washing was recommended by the paint manufucturer, to remove contaminants from the Slurry Blast & voids in the Keel Casting. The Keel would then need any surface rust mechanically removing before painting. I think the Phosporic Acid avoids this step, but am concerned about bonding.

John
Click to expand...


Ask the coating manufacturer !

I'd be concerned about un-reacted phosphoric acid remaining on the surface
 
It should be sand blasted to remove all rust then the first coat should be zinc rich epoxy paint, followed by three coats of ordinary epoxy paint. The first coat should be applied within 20 minutes of blasting otherwiswe you risk flash rusting if there in any humidity on the day.

If the contractor doesn't know that I would find one who does, otherwise the rust will be back. Or tell him he must do that. Obviously getting the first coat on would be faster if you help paint the first coat.
 
VicS said:
Ask the coating manufacturer !

I'd be concerned about un-reacted phosphoric acid remaining on the surface
Click to expand...

If my aged and addled brain is right phosphoric acid was sold to the car market as Jenolight and that needed washed off and dried before painting to remove the unreacted material. It was reasonably effective on older cars with decent quality steel
 
maxi77 said:
If my aged and addled brain is right phosphoric acid was sold to the car market as Jenolight and that needed washed off and dried before painting to remove the unreacted material. It was reasonably effective on older cars with decent quality steel
Click to expand...

Jenolite :D .. less addled
 
I used to use phosphoric acid for all sorts of things including rust control (well we had 100s of gallons on-site and always lots in the lab). It was many (many,many actually) years ago but I do remember that the ferrous phosphate formed as a loose white powdery covering. I wouldn't expect anything to stick to this very well. However, it should be easy to take off the loose top layer.

As far as I can remember, I often left the acid on inside tubes etc. as once the ferrous phosphate layer formed, the excess acid just sat there until something scratched off the coating and then it formed more phosphate when needed. However, you will be obviously be washing off the excess with water and also trying to remove loose phosphate.

I suspect that once all loose phosphate has been removed you will be left with a mono-molecular protective layer over the iron. That would help slow down further corrosion and provide a reasonable surface for priming. I guess the trick is to remove all loose phosphate without scratching the surface coating.
 
The phosphoric acid is used to " etch" to metal surface prior to bonding the epoxy
It should be rinced off and the surface air dried before painting
What I mean by " etch" is this --- imagine a glass smooth surface under a microscope , trying to attach paint to it .Bear in mind the keel will flex and change shape under load and experiance thermal stress.

Then etch it with acid , place under a microscope , what you will see will resemble looking down on a pine tree plantation from say 300 ft , thousand of tree like lugs with spikes and gaps so the paint will flow into all these knucks and crannies and form a micro physical bond .( without etching imagine looking down on a large flat concrete or Tarmac car park from 300 ft)
Several coats later and when set , it should form a air tight seal , impermalble to oxygen - hence corrosion Protection in theory .
Also the coating will not " shear " off when the keel flexes
Metal etches really well .
 
We are looking at a molecular well almost level
Imagine rolling hills , moors ( using a geographic analogy again) from say 5000 ft
That's what blasting has done - contoured the surface like hills , we are thinking smaller , you are more likely going to get a better bond from a hill coated in pine trees than a hill" your rough surface " coated in concrete
 
Your contractor is right up to the point about phosphoric acid. Your keel will almost certainly have a lot or pits and holes in the surface which will have salt in them. Blasting will remove the corrosion products on the surface and clean the metal it can get at. Unfortunately it will have the effect of forcing salt into even the smallest pit in the surface and that will restart the corrosion process immediately. Using slurry rather than dry blasting will help a little but not enough to remove the salt. Phosphoric acid will not be very effective as rust will have been removed on the surface but not in the inevitable voids. Acid will not convert the rust in the voids effectively. Acid residue will be seriously detrimental to adhesion of subsequent coatings as you suspect
Best process (and nothing is perfect on cast iron) is to pressure wash very thoroughly before blasting. Blast then pressure wash again as well as possible. This will cause surface flash rusting but it should be ginger, not black and is perfectly OK for most modern epoxies to go over. If you are able to a 2nd light blast will remove the gingering but probably not practical to keep the blaster hanging around all that time.
When the keel is dry - a hot air gun directed into voids will speed the process -apply a zinc rich primer. Epoxy or urethane will be best, then follow with the number of coats the manufacturer recommends and don't skimp on them. Finally antifoul.
Don't worry about the time between blasting and first coat so much as getting rid of salt as effectively as possible and then making sure the keel is dry. These are far more important than a little gingering
 
I can confirm that two pack epoxy sticks very well to phosphoric treated steel. My steel boat was treated 14 years ago and then undercoated with zinc primer/4 topcoats. The surface is perfect with no lifting or peeling. Very cheap and very effective. It has stood the test of time.
 
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