Figured I'd share, (and many of you probably already know):
I've been fabricating using hot-rolled 3/16, 5/32 and 1/8 bar or sheet for several years. For the longest time, I would scuff, prime and paint, and almost without fail, within a few years, I'd have rust coming through.
I found out about 2 years ago, that this because mill-scale is anodic, and in the presence of moisture will rapidly cause this rust. The solution; You must ENTIRELY remove the mill scale. That mill scale is TOUGH. I've blown through dozens of flap disks, and hours and hours of time trying to rid my fixtures of this.

Anyway, I came across a dude's YouTube channel the other day, and he had a cheap and less labor intensive way of tackling this.
Go to Walmart. Buy a 17 gallon tote ($12). Buy 6 Gallons of White Vinegar ($3 per gallon.) Fill the tote with vinegar. Fill the tote with fab'd parts.

Less than an hour after placing the parts in the solution, the mill-scale was rubbing off by hand. I waited another few hours just to be sure, then scrubbed off the parts with a bronze brush. The mill-scale was gone. Best part, you can re-use the solution for a while. (I'm told you can even filter the solution if it gets too dirty, then re-use.)

Also, does a decent job on rusty fasteners, but it takes a couple of days in the sauce to take effect.

Beats the h*** out of wasting time and flap disks.

 

Yes and coca-cola works well too!

 

6 Gallons of White Vinegar ($3 per gallon.) is a lot cheaper than 60 cans of coke($1.50 per can).

 

You've got to spring for the 2 liter bottles when they're on sale to keep operating costs down.

 

You can use muriatic acid (hydrochloric acid) if you're in a hurry, but do be careful: proper PPE and working outdoors is a must. Hydrochloric acid is a strong acid, meaning for a given concentration it is going to be more effective/faster (lower pH, meaning higher concentration of H+ in solution because the HCl completely dissociates into H+ and Cl- ions).

Vinegar contains about 5% acetic acid, that's the component that does the work for cleaning the metal. Acetic acid is a weak acid (incompletely dissociates, higher pH for a given concentration).

In both cases, neutralize the used acid solution with baking soda before disposal. Both can burn you, but hydrochloric will do it much much faster & severely.

You can buy steel in some dimensions that is sold as "pickled and oiled". This means it has already been descaled using hydrochloric acid at an industrial scale, then oiled to protect the surface.

 

lol. Good to know, but I hate to waste good CocaCola like that.

 

@GoinBoarding thanks for the data! I'll avoid the harsher stuff. I'm not in that much of a hurry. I have a pretty wife and two boys, so I have a fairly high-maintenance/low-extra-time home situation. Between the wife and boys, I have to squeeze time in for the project. I leave some of the "bits" in that vinegar soup for a week, and it doesn't seem to affect the metal, just the rust.

Regardless, if I ever find myself in a pinch, I'll look into the hyrdo-acid, with the provisos you mentioned.

Thanks!