First rinse was encouraging! No pictures at this stage, but I hosed it out and got as much loose stuff out through the drain as I could. I found I had only used half of my 240 mL bottle of acid, so the rest went in for round 2. Will take pictures after that comes back out.
Ew.
That is all.
Neutralize the acid before draining by adding baking soda and filling tank with water.
Good tip, pjbgravely. I siphoned out the first batch before neutralizing it (so I wouldn't weaken the second) and to my surprise, nothing happened! I think the rust neutralized all the acid for me. I am a little suspicious that the same thing has already happened to the second batch; I think if I were doing this again I would use more acid. Looking much cleaner though, and the second tank will be drained tonight. I will give it some sodium bicarbonate in the tank before the last flush, and probably another rinse straight from the hose. I am a little bit worried about all the water vapor that's been milling around in the tank for the last couple days, I think I might close with as much of a WD-40 spritz as I can get to and then filling with gas and a bottle of methanol.
So many chemicals!
Just dump a gallon of E85 in there and fill it all the way up with regular gas. That will take care of water.
Kenny_McCormic wrote: Just dump a gallon of E85 in there and fill it all the way up with regular gas. That will take care of water.
Which will turn the water into an un-burnable goop. E10 is probably what caused the problem to begin with. 100% isopropyl alcohol will absorb the water and is also burnable.
Blowing warm air into the tank is probably the best way to dry it.
Just because I was curious, I started investigating the particular effects of alcohol in the context of removing water from gas. I stumbled across an interesting tidbit here that indicates that higher aromatic content in the gasoline is more tolerant (able to absorb more, keeping it off the tank walls). Wikipedia research seems to indicate that higher octane gasolines have higher aromatic content, and thus should be a better choice for my first tank. Some fuel additives may add aromatic hydrocarbons as well, but I did not yet find any particular information about which and how much.
As far as alochols are concerned, the first link states that isopropanol is able to absorb more water on a volume basis than methanol. If the numbers are to be believed, it is around 10 times as effective! From that page:
http://yarchive.net/chem/gasoline_dewater.html said:
..."Why IPA, why not methanol or ethanol, which are usually cheaper?". The answer is dependent on two aspects, the actual ability to act as a cosolvent, and the effect of temperature on the phase separation point of gasoline/ cosolvent mixtures.
...
Adding 15% by volume of anhydrous methanol to a typical non-oxygenated gasoline will provide a water tolerance ( the amount of water that can be added before phase separation occurs) to around 0.16% at 15C, but at 0C the tolerance would be close to 0%... Adding more that 15% of methanol can adversely affect driveability...
Adding about 3% by volume of anhydrous iso propyl alcohol will provide a water tolerance of about 0.35% at 15C and about 0.2% at 0C, thus it's ability as a cosolvent is superior on a volume basis, and far less temperature sensitive than methanol.
Ethanol is between the two, with a 15% blend tolerating about 1% water at 15C. The best additives are the isomeric propyl and butyl alcohols, and above them, the tolerance decreases as the alcohols become less water miscible.
As far as my gas tank goes...a few hours with a hair drier dangled inside sounds like a pretty good idea! "There's no replacement for displacement" applies to water vapor as well.
In reply to pjbgravely:
Can you back that up with science? Or is it just more ethanol=evil hearsay? Don't get me wrong, the stuff has it's issues, both physically and politically, but it doesn't turn to goop a the drop of a hat, and it's hygroscopic nature is beneficial in this case.
Based on the information I posted above, I would guess that the ethanol addition to gas could be responsible exactly because it is hygroscopic. Specifically, this would boil down (pardon the mixed metaphors) to the temperature-dependence of its water tolerance. I don't have the figures for ethanol, but even IPA tolerates less when cold.
So, if open to the atmosphere (as I suspect my tank was), on a warm day the alcohol-gas mixture will absorb all the moisture it can from the atmosphere. On a cold day it can't hold it all, and the water falls out of solution to rust out the bottom of your tank. The more alcohol, the more it can catch and release in this way.
Of course, I'm speculating, but I'm highly trained in the speculative arts 
On the other hand, as long as I am going to use it warm and not letting in extra water from the outside, this hygroscopicity (if that's a word) should still work in my favor when trying to dry out the tank!
Again trusting the numbers from the sketchy usenet archives, E10 should have a tolerance of ~0.6% on its own. That is, at 15 C, I can dissolve 225 ml of water in a tank of E10.
...But I have half a liter of IPA. 500 ml IPA + 38 liters (ethanol-free) gas = 1.3% blend of IPA, which should tolerate 0.15% water, or 57 ml water per tank. Since we have E10 and not E0, I am not sure if it would add perfectly, but if it does I might expect a total tolerance of 0.75%, or 280 ml per tank. That's about a cup, so one should try to dry it out first!
If instead I used a gallon of E85 and 9 of E10, I would basically have a tank of E18, which should tolerate about 1.1%, or 418 ml. That is definitely an improvement in terms of absorption, but if the author was worried about 15% methanol screwing up your tune I am not sure how he would feel about 18% ethanol.
Of course, these numbers are pretty shaky, since the original figures were from memory to begin with and I am not terribly reliable with quick math.
The original link goes on to explain a procedure where by one could actually test these figures...but that, dear readers, is more of a pain in the butt than I am willing to commit to for this particular project. Another time, perhaps!
Kenny_McCormic wrote: In reply to pjbgravely: Can you back that up with science?
No just personal observation. You are correct the goop part takes a while.Ethanol plus water equals watered down ethanol. Ethanol draws water out of the air in the tank causing a very watered down ethanol after a few years to the point where you can see it in a glass jar separated from the gasoline. That is how you remove ethanol from gasoline. Pour in some water and remove the liquid at the bottom. There must be a reaction because what it turns into is a mess in 4 to 6 months.
I first started seeing this problem when E10 became main stream. When I would put away small engines for the winter I would drain the fuel, add new fuel with stabilizer then run the engine dry. This stopped working with E10. I would find after 6 months of sitting the bottom of the bowl lined with yellow gel, usually sticking the float down. After that I started taking the bowls off after running them dry to keep this from happening.
I finally figured out that the ethanol was combining with water over the summer and getting stuck at the bottom of the bowl. When I ran the engine out of fuel the mixture was simply stalling out the engine, then it turns into the gel slowly.
We finally have E0 gas where I live, and that is all I use in small engines. I no longer have to clean carbs or disassemble them so they would run in the spring. I know that most people do nothing to prepare there engines for winter. Every time I try it the gas varnished the carb and it has to be cleaned to run correctly again.
Most people I see run a lawn mower or such for 5 year then buy a new one. I keep things for 30 years, and that little bit of extra work makes it easy.
So I got good news and bad news.
The good news is, things looked muuuch better in the tank after the acid baths. At lunch today I decided a little more acid might be nice, hopped over to the store and poured in another 3/4 bottle or so of my good friend The Must for Rust. This is what it looked like when I got home:
Just the top shelf, nothing exciting here. But compare to the after-draining pictures:
That surface rust comes on fast after an acid etch, I don't care what it says on the bottle. Perhaps in the original concentration that is not supposed to happen, but whatever chemical the bottle claimed was supposed to keep that from happening swam away in the rinse. Still, much better than before! You can see the bad news around the periphery of the photo, though: big chunks of that old crap. It's hiding behind the baffles.
Ugh. Passenger side: (you can really see how full I filled the tank. That corrosion at the top is not my fault, though.
Look carefully through the left circular hole in the baffle and you can see the big rusty mess still intact, like the day I got it. Unable to hit it with the hose or brush, it has withstood the bath. The lighting in this photo doesn't really do it justice, but it looks like the same fluffy orange E36 M3 as you see making cameos in other photos in large chunks. Here's another shot:
Top down view:
Driver's side is tough to shoot from this angle:
Right now I've got the shop vac blowing air in to dry it out. After that, I think I'll see what I can get out with a 1/2" tube connected to the vacuum, but I am losing some confidence here. I think short of dropping the tank and shaking some chains in there or just replacing it, I could remove as much of the loose crap as possible and then give it another good long soak in a stronger phosphoric acid solution. But what if I still can't get all the big chunks of E36 M3 out? I can just picture it all heading straight for the fuel pump filter and putting me back where I started, but with a full tank of gas.
Whose idea was this??
On the bright side, if I may say so without paddling my own canoe, I have been very pleased with my new nitrile gloves! I finally ran out of my old VWR blues, and the new purple Halyard gloves have been working great through all of this. I usually tear through them pretty fast, but I think I poked a hole one glove during a full weekend of work, twisting fuel hoses with pokey hose clamps and all. Totally worth the extra $4 or so vs. whatever was cheapest on Amazon. So I've got that going for me, which is nice.
I went back out to the garage after taking them off (townhouse + neighbors = I can't dry things with a shop vac overnight 
) and sliced my finger opening on the tank opening, which is apparently pretty sharp! Purple gloves kept me from learning that lesson earlier.
In reply to pjbgravely:
It's far more likely that the small engines you use also use poor materials, so that scum you see are the fuel system parts breaking down. Why small engines get away with that is beyond me- it's not as if E10 is so new that they have no solutions to prevent stuff from breaking down. Car have been able to deal with it pretty well for decades- one needs many years of a partial tank for the water to cause problems.
The fuel may be the reason why the parts break down, but to me, it's the fault of the engine makers, since they should be robust to the most common fuel available on the market.
alfadriver wrote: In reply to pjbgravely: It's far more likely that the small engines you use also use poor materials, ...
When my small engines, and most of my large ones were made, the only country using ethanol was Brazil. They were built to use gasoline.
For the OP.
The metal will rust after cleaning if wet. The problem is that you can't get the water out fast enough.
Rolling the tank over with small round stones inside is and some kerosene is another method. People have built rigs to tow behind lawn tractors to make this easier.
Ummm, if you are going to drop the tank.... Should we start asking how much a new tank is?
pjbgravely wrote:alfadriver wrote: In reply to pjbgravely: It's far more likely that the small engines you use also use poor materials, ...When my small engines, and most of my large ones were made, the only country using ethanol was Brazil. They were built to use gasoline. For the OP. The metal will rust after cleaning if wet. The problem is that you can't get the water out fast enough. Rolling the tank over with small round stones inside is and some kerosene is another method. People have built rigs to tow behind lawn tractors to make this easier.
That must have been a long time ago, as the US has had E10 since 1988. Not across the entire country- that wasn't until 2007 when it became the majority fuel in the US. But it's been common enough that cars have been robust to E10 since 1990, so small engines should have been robust since then, too.
Before that, we had Methanol in our fuels.
This isn't new what so ever.
This Miata is robust to Ethanol. It's not robust to water, apparently.
You've also gotta keep in mind that gas was changing a lot at the same time E10 was being introduced nationwide. Most notably, it's way more volatile (IIRC whatever metric is used to rate this was halved), old cars vapor lock way easier now, and left in a carb vented to atmosphere it evaporates and turns to varnish faster.
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