Still perplexed with phosphoric acid

I'm still perplexed using Klean-Strip Prep & Etch to remove rust on my trailer. For the past few days I brush it on in the afternoon or evening and leave it on till late AM the next day. I find it a kaleidoscope of spots of black, streaks of white, and expanses of glossed over light brown rust. The instructions did say to rinse it off. Rinsing and letting it dry leaves me with spots of black, and equal amounts of white and light brown rust. The gloss is gone. I keep figuring at some point all the rust will be gone and I think I'm making progress. There seems to be more and more black and less and less rust but still quite a bit of white. What bothers me is there seems to be rust under the white.

Should I eventually see no rust?
What is the white?
What is the black?

I'm figuring now this trailer is rustier than what Klean Strip had in mind. This trailer had no paint. I'm sure it spent years if not decades outside w/o paint hauling wood on a small farm. It didn't have any flaking rust--it's not like it was buried in dirt with holes imminent.
 
how about trying a different brand rust treatment on an area and see if you get the same results. Napa sells a rust converter that you might want to try. Bill
 
The white is where the acid has worked on the non-rusted metal. Some may be permanent or a crust. Any loose residue can be wiped off. If you want it bright a wire wheel will shine it up. The black is where the rust has been converted and can be painted over. Preferably with a good epoxy primer. The light brown is flash rust on the bare metal. A wire wheel with take it off easily, or you can treat it again wait 15 minutes, rinse thoroughly, dry and apply Picklex 20, or just dry and keep it in a dry place. Paint within 48 hours or rust will begin to form again. Use a good wax/grease remover to wipe down the surface before you paint.
 
I use OSHPO brand phosphoric acid treatment. The instruction call out that you need to leave it on wet till the rust turns black then rinse it off. The black is iron phospate which neutalized rust. The white is dried phosphoric acid that has not reacted with the rust. The OSPHO that I use till do exactly what you are describing if you don't keep a wet coat on the work piece. After the rust turns black, rinse and dry. I also go over the entire treaded area with 180 to 220 grit paper after it has dried completely to remove the white residue. I have found that primer or paint will not stick to the white residue.

Good luck.
 
you have to keep it wet for a while - not just apply once and wait. I like a spray bottle.

Also, the stuff isn't magical, it's best used as a final step in preparation, after sandblasting, sanding, or wire brushing. Not as the primary means of rust removal.

Clean bare metal is always the best, use this stuff only once you've finally given up on trying to get that.

I don't wash it off with water, I find that just introduces a whole new problem with flash rust. Instead I quickly wire brush it.
 
Thanks, that's what I was beginning to see--it's intended use is the last step in painting prep. I'll just keep painting it on and pressure washing and repeating until I think the rust is all gone. I plan the final step to be spraying flash rust from the last washing with with spray cans of Rustoleum rust reformer which, in spray cans, DOES contain paint as well as the reformer. It leave it "painted" flat black. I plan on that being my primer and it'll be a month or two after that before I can actually paint it.

I like electrolysis better. Just leave a piece in solution and even the worst rust is removed. I still get flash rust but I can clearly see that's all it is.
 

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