Rust (streaks?) in steel tanks

[TrimixToo]

It gets tested for Grade D breathing air.
My HP compressors have a lower dew point, but it is plenty dry for scuba use.

Grade D's dew point is -50 or better. That's pretty dry!

 

[SubseaSports]

so I did a bit more search, and found "official" recommendation for cleaning with 5% phosphoric acid, rinse with hot water, then blow dry with compressed air from a cylinder. As soon as I receive the phosphoric acid, I'll redo the cleaning and see if I can avoid all flash rust.

But I'll be taking it to a different shop. It will take a bit to set up a VIP course.

Sorry for your hassles...smh

You may have omitted this step in your post but if not, I would add a wash step before the water rinse to remove the phosphoric acid. Simple Green works, I even used Dove dish soap.
Very important to remove any trace of it [acid].

 

[wetb4igetinthewater]

Sorry for your hassles...smh

You may have omitted this step in your post but if not, I would add a wash step before the water rinse to remove the phosphoric acid. Simple Green works, I even used Dove dish soap.
Very important to remove any trace of it [acid].

At what concentration?

 

[SubseaSports]

You want enough to make suds. I start with about 1/4 tank full water and soap. Of course you need to aggitate it for it to do its job...I tip end to end several times. It will be handy if you have a plug for the tank. Its a bit if a work out, hopefully this isnt a 133 or a Heiser lol.
Dump appropriately, then start the water rinse process. Rinse til you don't see suds. I aggitate the tank in this process as well.
I see you have a stand which is nice.
I don't, but lay tank sideways on chair and use a LP hose on an air tank. I work the hose in and out, rotate tank half way and repeat. Done.
The pvc pipe works, but if it is close in diameter to the tank thread diameter you limiting how much can come out of the tank.

Also, do you tumble the tank when doing the acid treatment or let it sit? Or....?

 

[sprockjohnson]

You want enough to make suds. I start with about 1/4 tank full water and soap. Of course you need to aggitate it for it to do its job...I tip end to end several times. It will be handy if you have a plug for the tank. Its a bit if a work out, hopefully this isnt a 133 or a Heiser lol.
Dump appropriately, then start the water rinse process. Rinse til you don't see suds. I aggitate the tank in this process as well.
I see you have a stand which is nice.
I don't, but lay tank sideways on chair and use a LP hose on an air tank. I work the hose in and out, rotate tank half way and repeat. Done.
The pvc pipe works, but if it is close in diameter to the tank thread diameter you limiting how much can come out of the tank.

Also, do you tumble the tank when doing the acid treatment or let it sit? Or....?

This is similar to what I do as well. I use a mild degreaser like Simple Green/ Mean Green and hot water, dump, and then do my final rinse with boiling water followed by blowing it out while upside down with dry low pressure scuba tank air. I agree that the blow-out tube needs to be smaller in diameter to allow the moisture to blow by and out of the tank.