Cleaning the fuel tank need your opinion.

Phosphoric acid is also known as "metal prep", a etching treatment for bare metals before paint work. It lightly eats the surface of clean steel to "roughen" the surface allowing the paint to stick better.
 
Nice job so far. Looks like you got all that gooey crud out. Could you tell how much the gunk was removed by the MEK vs the Oxiclean, or vice versa?

Some of those rust spots appear to be a bit deep (difficult to tell in photos). No leaks? If not then check again after the acid treatment.

Phosphoric acid is a relatively weak acid (especially at that dilution) so it's action is slow. I think I let mine soak for several hours and still had to do it again. But I did not use another cleaning agent before the acid (as you did), so that would have slowed mine down. However don't be afraid to let it soak for a couple hours. The acid solution can be reused multiple times. Therefore if it still needs more cleaning then just pour it back in.

After the acid you'll need to rinse it. As much as I hate to introduce water to a freshly exposed metal surface, it is the easiest way with a large tank like this. You can flush it with a garden hose for a few minutes. Then make up a solution of water and baking soda for the final rinse. Do all of your flushing consecutively and immediately follow it with some denatured alcohol to absorb any remaining water. If you don't have denatured alcohol then try using a leaf blower to evaporate the water quickly. It will hide in the seams and crevices so dry it well. While the phosphate film that results from the phosphoric acid will act as a protectant, it isn't perfect. It results from the reaction of the acid and existing rust. Therefore areas that had no rust won't get the phosphate reaction and rust is always a possibility.
I believe most was removed from the MEK due to the thick layer of varnish/guard left in the collection can. As to the Oxi Clean, I use the power washer when raising that out after letting it sit over night. I didn't get a good look before I power washed. I did flush out the tank several times after the Oxi Clean and was pretty brown at first, could be Rust or could be more curd.

The acid will be reduced in concentration due to the conversion of the Rust to Iron Phosphate. I was never good in chemistry so I don't know how much of a reduction will occur.

I plan to shake, blow, absorb up as much of the final rinse as possible then will leave the tank in the sun for the day. It will get pretty hot being black which will help evaporate the remaining trapped water. Last rinse with baking soda is a good call as is using so denatured alcohol. I'm think of also running a old hair drier into the tank to help dry it out. Looking to get the tank back in soon then putting some fresh gas in it.
 
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I recently cleaned-de-rusted the gas tank from my Alfa Milano sedan. Can't say if how I did it is better or worse than the other suggestions, but it seemed to work for me:
1) Slosh with "Evapo-Rust" (done twice), gallon jug obtained from Harbor Freight store. Accompanied by much shaking and flipping of tank to get the rust to fall out of the bottom supply and top sending unit holes.
2) Second cleaning/sloshing with water/detergent-degreaser mixture.
3) Final rinsing (done twice) with old gasoline.
 
Finished the Aicd wash with 40% Phosphoric acid. Here is the final results. Was hoping the surface rust would not come back so quickly, it is still a little damp in this picture. Dried it out with a rag on a stick to mop up the water then ran a hair dry into the sending unit hole for about 20 minutes.

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Not sure about the chemistry either, but I know from using POR's Metal Prep product, any bare metal that comes in contact with Metal Prep for the prescribed amount of time will look like the grayish-whitish areas of the inside of your tank.

The instructions that come with Metal Prep say that even Metal Prepped metal can flash rust after being rinsed, and that is the very light colored rust that we see in your tank. I don't think that's a big deal.

Since the acid is supposed to convert the bare metal and rust to a phosphate, the only conclusion that can be drawn from the last pic is that wherever the rust is that darker color, that conversion did not happen. Maybe this is telling us that the areas that are still dark-rusty did not get cleaned well enough to allow full contact of the acid with the metal. If that sounds reasonable, maybe another round of degreasing to really get it clean and then another round of acid (or a commercial product like Metal Prep) is called for.

Or you could go right to a commercial product at this point: https://por15.com/collections/fuel-tank-restoration/products/fuel-tank-sealer
 
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When I first empted the tank of the acid, it was gray and white with no rust. Riased first with a Water Baking Soda mix then washed out with a hose. Flash rusted pretty quickly before I got it completely dry.

So as Dan pointed out do I use a Tank sealant or just fill it with gas and look to the fuel to act as the Rust inhibitor? If I do go the route of a sealer is the POR15 the best to use?
 
Since the acid is supposed to convert the bare metal and rust to a phosphate, the only conclusion that can be drawn from the last pic is that wherever the rust is that darker color, that conversion did not happen. Maybe this is telling us that the areas that are still dark-rusty did not get cleaned well enough to allow full contact of the acid with the metal. If that sounds reasonable, maybe another round of degreasing to really get it clean and then another round of acid (or a commercial product like Metal Prep) is called for.
That grey color is the phosphate that results from the conversion process. Ideally the inside will be that greyish color over most of it, but it will never be 100% [see the note at the bottom of the post about this*]. As Dan says, if you did not get a good change in color then it may need more acid treatment.
Although......


When I first empted the tank of the acid, it was gray and white with no rust.
So it sounds like you did get a decent conversion and should not need further acid treatment. As for the flash rust.....


Flash rusted pretty quickly before I got it completely dry.
That's what I've been saying, it is critical to get it completely dry immediately. One way to help achieve that is use of a highly hygroscopic agent such as acetone or denatured alcohol. Most tank coating treatment kits include that as the final step prior to applying the actual coating product. It is difficult to properly dry the inside of a large enclosure like the fuel tank.


do I use a Tank sealant or just fill it with gas
I don't think you need a sealer in your case. They are better suited to more severe rust conditions where the metal has developed pin holes or very thin/weak areas. The sealant then acts as...well...a sealant.
Either just use it as is or if you like you could try another round of acid followed by a better water removal process to help avoid the flash rust. Then as soon as possible (the sooner the better) fill the tank with fuel to keep it rust free.


then another round of acid (or a commercial product like Metal Prep)
Just for the record, Metal Prep is phosphoric acid. ;)


[* NOTE: Earlier I recall someone saying that only rusty areas will react with the acid and change the color of the metal. That isn't correct - it does the same for non-rusty metal as well. Although it isn't as prevalent on bare metal as it is on rusty metal; the rust causes a stronger reaction and therefore a darker resultant color. The areas that don't react and change color usually have some sort of film like grease/oil or mill-scale that is preventing the acid from reaching the metal's molecules sufficiently. Either prolonged acid treatment time or a good cleaning of the surface before will help. Sometimes that may mean a more aggressive removal process like surface blasting or even a stronger acid like diluted muriatic first, followed by the phosphoric acid. I've had to do this on vintage motorcycle tanks that were extremely rusty - muriatic acid first to really strip it, then phos acid to convert it.]
 
Thanks Jeff, decided to skip the tank sealant after reading up on some failures. The rust is 98% light surface rust so I think I am good. It boiled down to new tank for $180 ship or just reinstall. It looks good enough to keep. Now if the USPS can deliver the tank sending unit and prefilter I an set to go. Should have spent the extra $7 for FedEx, you would think the USPS would try to get their act together and try to be competitive with UPS and FedEx while they still have some business.
 
Shipping is definitely a issue these days.
Sounds like you are good to go with the tank. A little light surface stuff won't be a big issue once it's got fuel in it all the time.
 
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