Mill scale is the black gunk that coats mild steel when you buy it (what you have is actually hot-rolled mild steel. Cold rolled mild is far rarer and more expensive to find, but does not have this black coating).

You will require:

• A non-metal liquids container large enough and deep enough to completely cover the armour article you are placing in it.
• Enough cheap, supermarket bought, plain vinegar to completely submerge the armour articles when they are in the aforementioned container.
• A source of nice warm water.
• Some fluffy towels that you do not want to use for human skin again.
• A bit of patience.

Now you are ready to place the armour in the container (it helps if it is resting on something like two drinking straws, so that the vinegar can get in underneath. Otherwise it will take longer as you need to turn the armour to allow vinegar to reach every bit.

I prefer to strip the coatings of metal at the stage I have cut out my patterns but before I shape or weld them. This means I can get a lot of steel soaking at once. If your armour is already built, then so be it.

Note: vinegar is BAD for leather. You WILL need to remove all strapping, and replace it after polishing.

Completely cover the armour in vinegar. If any part of the metal protrudes above the surface, or if there is other steel lying around too close to the container then it will corrode and die like nobody's business. The interface between vinegar and air is highly damaging and will even rust stainless. Anything below the surface is going to get quietly stripped of all surface coatings.

About 48 hours of quietly bubbling away to itself (you may wish to stir it once or twice over the course of time. It gets you a cleaner result)

The steel will come out white and a little powdery. Give it a soak under some warm water (or a wash under flowing water, same diff) and then dry carefully. This is the stage that the metal is particularly vulnerable to airborne corrosives.

Buff on a wheel of either stitched denim or loose calico as soon as possible using either green or white buffing compound. This will give you a protected bright polished finish that is better able to resist corrosion.

Use a polymer based wax product like "Nu-finish" or turtle wax to further protect your armour.

Under no circumstances should you use a drill- or angle grinder -mounted wire brush to take off rust or mill scale.
You will permanently damage the surface and increase the surface area available to moisture and corrosives. The fact that the metal in the wire brush is different to the mild steel you are working will also start electrolysis, a process where the two metals act as a low powered battery and further increase discolouration.