Motorcycle Tank Rust Removal with Evapo-Rust

Before:

You can see a thin layer of red rust on most of the inside.

After:

Much better! Almost all of the rust is gone. You can see some pinkish paint overspray on the inside of the tank, don't confuse that for rust. Whoever painted this tank must not have masked off the filler opening. The red oil in the bottom of the photo is some Marvel Mystery Oil I used to prevent rust.

What I used

I found this stuff called Evapo-Rust that had good reviews. I had used it on a couple items and was very happy with the results so I decided to try and use it on the inside of my Mondial gas tank.

The manufacturer of Evapo-Rust claims it is relatively safe. This is from their website:

"Evapo-Rust® rust remover is safe on skin and all materials except rust! It's also biodegradable and earth-friendly. Water soluble and pH-neutral, Evapo-Rust® is non-toxic, non-corrosive, non-flammable, and contains no acids, bases, or solvents. Evapo-Rust® is simply the safest rust remover."

They also claim is is safe on paint:

Powder coating and paint will not be removed as long as the paints do not contain oxides.

However, NOTE the exception they mention about oxides! Some paints do contain oxides, particularly red paints. Use care, especially around red paint.
I also saw it damage the clear coat on some of my decals. I was able to clean this up on my tank without trouble, but you should be careful!

My Mondial gas tank is 16 Liters or 4.2 Gallons.  Conveniently, the Evaop-Rust is available on Amazon in various sizes:
Evapo-Rust 1 Gallon

Evapo-Rust 5 Gallon Pail

Other items I used:
90% Isopropyl Alcohol to help rinse out the tank.

Marvel Mystery Oil to coat the tank after cleaning to prevent rust.

How I did it

I took the tank off the bike and removed the petcocks. I've heard people have done this with the petcocks installed but I didn't see any advantage to trying that. I made two petcock hole plugs out of a pair of bolts that I shortened and some rubber washers. I also used some teflon tape to help seal the plugs.

I put some plastic down and used a aluminum turkey roasting pan to catch any drips. I also put some blocks under the tank to hold it off the bottom of the pan. I made sure the top filler of the tank was perfectly level so I could fill the tank completely.

After filling the tank, I made sure the outside of the tanks was completely clean and that the was no evapo-rust on the outside. I didn't want any chance of damaging the paint!

And then I left the tank for a couple hours. I'm not sure exactly how long, but it was more than 2 hours and less than 4.

I've seen metal "flash rust" quickly after de-rusting, so I really focused on completing the next steps quickly.

When I decided the tank was "done", I emptied the tank into a large plastic bin so I could recover the Evapo-Rust for future use. And the I immediately washed the outside and inside of the tank with water. (It was really nice that the Evaporust is biodegradable, minor spills were no concern!) 

After shaking the tank out I put in about a cup of 90% rubbing alcohol and shook it all around and then drained that. And then I repeated that again with another cup of alcohol. The idea is the alcohol blends with any remaining water and then dries out quickly. 

But I was still concerned about flash rust, so after shaking out all the alcohol I could I immediately poured in about half a cup of Marvel Mystery Oil and shook and tilted the tank every possible way to coat the entire inside. I poured out the oil and repeated this step again with fresh oil. After the second oil coat the excess oil poured out looking nicely unpolluted with water or alcohol.

(Note: I wouldn't count on Marvel Mystery Oil for long term rust proof storage of an empty tank. But for short term use before filling with gas it is good. Marvel Mystery Oil is commonly used as a fuel additive and it a pleasing peppermint smell.)

Results

I think the tank came out very well. Most of the tank looks great. That might be a bit of rust left in the bottom, I probably should have left it a bit longer or maybe rinsed it once and refilled for a second Evapo-Rust treatment.

I'll be curious how long this lasts. Some say they you should use a treatment that leaves a coating to protect the tank from rust, they tend to recommend a phosphoric prep and etch. I have done that in the past, but dealing with the acid is very hazardous! This Evapo-rust is much easier. As I said, we will see how it lasts!