Opinions before I paint

Hello!
I have to break my I.H. U-240 to fix a clutch issue here in a day or two. In a weeks time I plan to have the chassis primed. The chassis has a coat of Ospho brush applied. How much of the chalky residue needs to be removed? Most of it comes off with little effort with a wire brush, how picky do I need to be here? Next, I plan to use PPG or Dupont epoxy primer on both the chassis and the sheetmetal. Some of the sheetmetal was etch primed several years ago and there is some light rust shining thru...I had considered using the Ospho on the sheetmetal but read that it leaves the surface of the tin a little rough. Is the Ospho worth the effort on the tin or just stick with the etch primer?
I'm not building a show tractor but something close to it...I want it as nice as it can be.Thanks for the advice...
Dave

I can't recommend phosphoric acid on cast as it gets in the pores. You may want to wash it with soap and water if possible and dry it before you get flash rust. It may take several washings, then use repeated applications of wax and grease remover. Either sand the rust off the sheet metal or use a fiber brush on an electric drill, which is much more effective. It will leave deep scratches that can be partially removed with 150-180 grit sandpaper. If you are afraid of rust use Picklex 20, which acts as a preventative and also gets rust in the pits, if any. It will leave some residue that can be sanded off, then use wax and grease remover. Don't use etch, use two coats of epoxy primer followed by sandable surfacer, which will fill the scratches. Absolutely do not use etch on the cast. The amount of phosphoric acid left will combine with the etch and likely will lift the paint. No I don't use ospho, it is too strong, Picklex is a much better and safer product -- but I don't use it on cast either.

If it were me I would have sandblasted down to bare metal and just used epoxy primer without the Ospho. Since you have it on there all you need to do is wire brush it off and blow the residue off with a air hose and then prime with epoxy primer on the body and sheetmetal. Just get a data sheet on whatever primer you use and stick with the directions. Epoxy primers have a recoat window you will need to pay attention to, but the Dupont epoxy primer gives you 72 hours to topcoat.

CNKS, I had started using etch primer on the cast several years ago. I had three kids go off to college back-to back and the tractor project had to go on the back burner for a spell.I pressure washed all the etch primer I could off of it and degreased it before appling the Ospho.I applied the Ospho not quite a week ago. Do you think I'll be okay at this point or should I go into damage control mode? If so, what would the remedy be?

You were right in applying the Ospho. My experience has been that anything that I have had sand blasted has had little spots of black here and there. That is rust. Right? I have had body panels with small pits professionally prepped and painted and after a few years, the paint is lifting in small bubbles from those pitted areas. You don't want that. Right? A little white residue will be left behind in pores/pits after washing, but it is loose whereas rust still down in those pits/pores is bonded as it is still working on the iron and steel. right? So your epoxy paint can get down under the particles of residue and bond to the metal but it can't fully penetrate the working rust. The residue is what is left after the phosphoric has done it's job of eating all the rust and converting it to inert iron phosphate which is in itself an excellent protective coating. So go ahead and pressure wash and brush to remove the worst of the residue, let it dry for awhile, then go over it with a abundant application of your favorite prep solvent to clean and remove moisture to prevent flash rust. I have left bare metal for days or months with no flash rust occurring but this will vary geographically, so you may need to hasten to apply primer or do it at your leisure. I have never had to worry about moisture left over from washing. I get on my soap box about this because I have to keep looking at those little bubbles on my tractors, and I don't want it to happen to you. In addition, you don't need to spend the long dollar for Ospho or Picklex or any of the many others. You can go to any dairy supply or restaurant supply and pick up gallons of phosphoric for about $10.00 per gal.

I cannot say for sure if the ospho will lift the paint, If mine I would repeatedly try to wash the stuff out of the pores, you can neutralize it with baking soda then remove the baking soda. with water. Final step is wax and grease remover. That is only my opinion, others have conflicting opinions. With the different opinions, you have to form your own. But, I can assure you that what I say works, and has worked for the last 10 years. I am not conceded, that is a statement of fact. I have never had rust work its way through paint. The first tractor I did had rust on the hood after sanding, it was neutralized with Picklex 20 and the residue sanded off and cleaned with wax and grease remover.

I use Ospho and have painted over it the enxt day, I have never had a problem with it, been doing it for many years, But if I use lacquer primer it wont stick unless I sand it first, Will epoxy primer be what I should use?

Lacquer primer is one of the worst as far as adhesion. Epoxy primer is the best and can be used under anything, or over anything for that matter. The lack of adhesion is stated in the instructions for some lacquer primers that say sand AND use etch primer before applications. If you get by with ospho or any phosphoric acid product on cast ok, but I'm not going to try it. Mainly because I remove all the rust I can with a wire wheel on an angle grinder. IMO any rust remaining in the pores of the cast will be sealed in by the epoxy. Some may disagree with the last statement, but epoxy is as good as it gets. The only paint lifting I have seen occurred because I messed up with the application one way or another. I seldom use any phosphoric acid on sheet metal because there is zero rust when I am done. If there is any "microscopic" rust, the epoxy takes care of it. If I ever run into any (the hood on my Super M is badly pitted) I will use Picklex 20 and sand off the residue before it is painted. Picklex does not have to be washed off.

I dont use it on cast either. I will get some epoxy primer, Thanks for the advice.

Thanks for the information fellas...Its been 20 years since Ive done any painting so this new stuff is Greek to me.