This was posted by bluebikerblan on another site:
Gents - In addition to lot of bikes & lots of years, I'm a degreed chemist (retired) and would like to add my two cents re the acid/electrolysis/coating discussion. Primarily with a concern
for safety in using any of these chemicals. Here's a long standing write up from another forum
that's tried and true in my opinion on using a diluted form of phoshporic acid which is widely used in the Dairy & Food Services Industries called MSR (Milk Stone Remover) which is comparatively safe to use, readily available and comparatively inexpensive.....Best, Blue
Gas Tank Cleaning with Milk Stone Remover - MSR
Rust Removal with Phosphoric Acid based 'Milk Stone Remover'
This tip is from another forum.....I too had mixed results using electrolysis and Kreem and was better pleased with MSR (Milk Stone Remover) and a tank lining product called Red Kote which used to be red but now is blue in color. The tip is written by Bill Lane and posted on a CB 45O forum
"MSR is about 70% phosphoric acid, with an orange dye. Mix it 50% with water, fill the tank, come back later. It will save even the worse tanks. It's hell on rust, won't touch chrome. It will mess up paint if left long enough. Some tanks will get de-funkified in a few hours, some may take much longer - days or even a week or more, if they're really nasty. Those of you who have used Naval Jelly to remove rust are already familiar with how phosphoric acid removes (reduces)
rust by reducing red iron (ferric oxide) to loose black (ferrous) oxide and iron phosphate (also gray-black)
It (MSR) is also way less dangerous than muriatic (hydrochloric) or sulfuric acid to work with. If you get some on you, at least you have a chance to make it inside and rinse it off before it burns the livin' snot out of you. No noxious fumes either. I'm a degreed chemist, I've seen what acids can do, so I stay with MSR, it's much safer to work with.
I've also had unsatisfactory results with electrolysis. It works "line of sight", meaning that if the sacrificial electrode can't "see" the area nothing will happen . That means that convoluted volumes or parts (like tanks) will yield indifferent results. MSR also works better on flat pieces. I keep a big 60-quart cooler full of MSR solution which I process nearly everything with it. Some people claim it leaves some sort of protective coating that prevents further rust. I have not found that to be the case at all. In my experience, you can see the rust re-forming before your very eyes. So, you need to quickly either paint the piece, spray with WD 40 or light oil. For a tank case, fill quickly (or swish around) some heavy 2-stroke gas/oil mixture to coat it while you prepare to do something else with it.
Everyone has a favorite tank "sealer" - I prefer Red-Kote myself, use what you prefer. Although Red-Kote has changed recently, it's now blue, but the same stuff. If you go with Red-Kote, mix it down a bit with acetone to make it easier to use. The stuff is really viscous out of the can." -endquote-
Other tank lining products are POR 15 , which a lot of people like as well. And Kreem, both are products I have used with difficulty and mixed results.
Milk Stone Remover, MSR, is available from a variety of Dairy Service products suppliers. I found and purchased MSR from a localTractor Supply store.
Hope this helps with a rather messy and onerous task at best, It did for me and was clearly the best to use of the bunch for me. Best, Blue