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It´s the same situation after the rust removal, no oil, only rinse mineral spirts ( pure alcohol ) or pure undiluded car window antifreeze liquid.

Scirocco's comment triggered a comment, namely don't use any WD-40 if you play to coat the tank in the end. WD-40 has oil in it that will harm the ability of the liner's ability to adhere to the metal.

After I read the post the second time, I picked that up. I do not plan on putting a sealer in the tank. I will also speak to my painter before I use anything in the tank after the final rinse. I do not want anything to bleed out and ruin his painting efforts. I also want to avoid any flash rust, but with my experiences with phosphoric acid solutions that is usually not an issue. the metal can last up to a week or two without rusting after treatment.

Good news! After soaking in multiple positions for 24 hours, there are no leaks! I am very relieved not to have to deal with any weld repairs. I think I have all the surfaces covered with the positions I used. I was working with only 1 gallon. The only area I am concerned about is the top of the rib down the middle (to clear the main backbone of the frame). Not sure I had enough volume to submerse that area. As soon as it is drained, I will post up some pics of the results.

Just getting a first look down the filler. As I thought, you can catch a bit of orange on the center rib. As soon as it's dry I will get some deeper bore scope shots. Definitely better.

After getting the tank dry I got out the bore scope. Not what I wanted to see, but the seams look better than expected. There is that one corner, though, that gives me pause. (Last pic)

Nessism wrote: www.homedepot.com/p/Klean-Strip-1-gal-Co...-GKPA30220/100406369

SDS says 35-45% phosphoric acid. You can cut it with water and it will still work, just a little slower.

Alright, after two attempts I am going to give this product a try. My Evapo-Rust has been weakened from use, and I didn't clean all the gunk out before trying it. The POR15 didn't perform well enough after the Cleaner Degreaser (which worked great, by the way). This product is less expensive than buying fresh Evapo-Rust and it is in stock at the local store. We will see if another brand of phosphoric acid will do any better.

I do recognize the progress I have made in cleaning out this tank. It is much, much, better than when I started. I am hopeful that a fresh batch of treatment will get the rest of it out. Really happy I haven't found any pin hole leaks!!

As someone that's gone down the tank cleaning rabbit hole several times I want to applaud your effort. Your tank was in an amazingly sorry state and you have come far to get to where you are now.

One possible issue as I see it: there seems to be rust/crud spots down into pits in the metal and if there is any varnish covering over the pits that reduces the ability of the derust sauce from reaching the oxide. You might want to shoot a can of spray carb cleaner into the tank so see if it loosens any of that brown stuff up. If it does you then will have to attack the varnish one more time before going back to the phosphate.

The phosphate will start to leave behind a white chalky coating in the tank as the dwell time increases. I can already see that in places in your photos. It's not a problem per say but obviously, you don't want too much of that. I've used aquarium rocks before in the tank and shook the hell out of it to help scrub some of that off before.

Lastly, your tank is definitely borderline in terms of a candidate for cleaning alone vs. cleaning and coating. The POR-15 coating paint is good stuff and I wouldn't be afraid of using it, particularly since your tank was so rusty and the metal is likely weakened in places. I've got some tricks when it comes to coating so PM me if you decide to go down that road and I'll share.

One other thing, your tank has rust around the filler opening and no doubt, by the tank badge bars and such. You might want to strip all the paint off the tank and derust it completely before sending it to your painter. Sometimes painters don't want to fuss with such things and that rust may get painted over instead of removed.

My plan, once the rust is completely removed, is to put all the plugs back in and bead blast the entire outside surface prior to going to the painter. Rather than dump more money into buying additional cleaner/degreaser, I will try this new phosphoric acid. I am picking up 3 gallons, so I should only have to soak it in two positions to get full coverage. I am going to let each position soak for 24 hours before draining the tank again. Rabbit hole? You betcha!!

Far out Skid, a lot of money spent on tank restoration. when do you think it will be finished?
Put fuel tap and fuel sensor back in and fill with fuel. Done!
Hey, no one is gonna be looking in your filler neck are they!