Just finished cleaning 8 tanks, of various degree of a rust. nothing major some rust spots on a bottom and some flush rust.
I have build a nice tumbler tumbler and a whip, never was able to clean it as a whistle though
This time around i used, Esprit Rust Remover. (check ebay i bought 1/2 gallon of concentrate), it some non acid stuff, organic and non toxic.
(i used last year , but got black rust on a bottom of cylinders, so i decided to modify process a bit)
I got such of nice results , i like to share my process.
(i bet you can just roll tank on a ground without tumbler)
1. Made a 20% diluted mix from concentrate.
2. Filled tank to 1/2 let it run on a tumbler for ~30 minutes, this would wet the walls and remove flash rust
3. Put tank vertically , and let it stay for 24hr (again this is not acid, so safe to let it stay as long )
4. Use whip with solution in the tank to whip a bottom and sides, so black residue is not sitting on the bottom.
5. Put tank back on tumbler for 30 min
6. Pour solution out to re-use again (it is usable until jet black)
7. TWICE wash tank by filling half of it with tap water and rolling on a ground.
8. i cut off low pressure hose, connected it to second stage and regular lp tank full of compressed air.
9. Put hose all the way in a tank you cleaning, its kind of hard to hold 120cf tank upside down
this is what gym is for i guess
10. i emptied whole tank of air both tanks became very cold.
My tanks and water were pretty warm, due to weather, so if you air dry warm tank while it is warm it will get some flush rust.
So while tank is CHILLED do these:
11. while tank is cold, pour half tank of solution into it, and roll for 10minutes.
12 . repeat 7-10
Have a nice clean tank.
No rust, no spots.
