Lining gas tank - experienced?

Use electrical/chemical method to turn the rust left in the tank to black oxide.
Black oxide does not allow the rust to restart.

Battery charger, pool chemical and 12 to 24 hours, search the archives for the method.

Here is one

Jason’s de-rusting method

Hi all, I've gotten a few requests for the magical gas tank derusting
process. Wonder if it deserves space in the faq?

You need
1 rusty gas tank
1 small container of sodium carbonate, chemical compound, Na2CO3 pool/spa supplies PH+
1 4amp or better battery charger
1 average bolt approx 3/8 x 1.5
1 short length of copper wire, ~ 12 awg
1 qt naptha
1 qt denatured alcohol

Empty out all the gas, take out the petcock and remove the fuel cap. Remove any
old fuel residue and varnish with a good rinse of naptha, make it
petroleum free and then drain / dry. using a sock with a handful of small
nuts/washers etc, add a little water and shake this all around inside the
gas tank to loosen the big chunks, rinse with clear water scrub well. In a
decent pail (not oil drain bucket), dissolve some of the sodium carbonate in water,
1/3 cup for say a magna, 1/2 cup for a sabre in a gallon of clear water,
when its dissolved all the way stir it a bit more!

Seal up all but the filler cap opening on the top, pour in the well stirred
mix using a funnel if you've had too much coffee. Fill the tank right to the
top with water, set the tank so that the cap opening is the highest part,
burp out as much air as possible and keep the tank full for the process.

Wrap a half dozen or so turns of the wire around the bolt to hold it secure,
twist it tight so the bolt won't fall off. Attach the (-) lead of the batt
charger to the outer shell of the tank, attach the (+) to the other end
of the copper wire, suspend the bolt in the solution and turn on the battery
charger to a fairly high rate, an amp or two flowing is good, then wait.
Time to process is ~48 hours and it won't overdo itself.

The bolt gets nasty after a day, I cleaned mine now and again to remove the
crud but not sure it helps. After a couple days, remove the leads,
discard the bolt, save the wire, drain and rinse the tank well, drag out
that sock and slosh it all around to remove any loose material.
Rinse a few more times until the rinse water seems clean, shake well and
then use a bit of the alcohol to fetch out the rest of the water.

Once the tank is dry, you're all set to put it back into service.
More coating not required

Science, maybe remove this junk The process by which rust forms is
electrochemical in nature so this method employs a reverse current flow
in an alkaline bath at a higher voltage to reverse the process at a quicker
rate. There are actually two forms of rust: iron III oxide or red oxide
(Fe2O3) and iron II, III oxide or black oxide (Fe3O4)(FeO). Black oxide
is a smaller molecule. The electrolytic process converts red rust to
black rust and in the process the black rust becomes weakly bonded to
the base metal. The black rust that takes the place of the red rust can
be easily wiped, washed, or brushed off leaving rust free base metal.
Any pitting that has occurred will remain, this method will not repair damage,
but the pits will be rust free.

This is an alkali process and not acid, so you don't have a lot of pits
in the steel filled with stray hydrogen ions which would just love to start
rusting immediately instead of a much less active coating of black oxide.

-Jason