I'll share my experience.
Light rusting on blued steel. The very competent person from my local shop walked me through the procedure and had the products (less than $20.00).
Flat - sided profile steel wool (S/S, I believe). It looks like fine Christmas tinsel crumpled up, NOT like standard steel wool. Lubricate affected area with Kroil or CRC Knock 'Er Loose (letting stand for more penetration beneficial). Rub lightly with the steel wool, and be gentle on corners and sharp edges. Proceed slowly, and wipe oil to check progress.
Afterwards, when I was satisfied with the cleaning, I wiped the entire surface with 70% alcohol to remove ALL traces of oil.
Next, apply 44-40 Instant Blue. Use small rag or patch to wipe on thin layer, then wipe to remove immediately. Repeat until color match obtained.
44-40: to see its effects, dip a nail or knife blade into it. It turns it black. When I removed the foil seal from the bottle, the tip of the blade blackened instantly. This gives one an idea of how it should be applied.
Regarding the use of steel wool, I first cringed at the thought. However, it was very effective and removed remarkably little of the original blueing. No noticeable scratches were left on the surface. Very surprising, and I was a bit aggressive (relatively speaking) on the main area of concern. The explanation is that only the flat sides of the (tinsel-shaped) wool contact the flat surface of the metal.
The penetrant seems to be more effective if allowed to soak in, but all traces of oil must be removed prior to bluing.
Hope this helps. I am new to this, but I am very satisfied with the outcome.
Monty