Soak it in a baking soda bath to neutralize.
+1 Remove the headset, although I did not on the last bike I treated (I now have a headset tool kit). Acid will corrode aluminum, so if your headset is aluminum, it will affect it.
You should have a very dilute solution of oxalic for soaking.
From the vintage BMX site:
"Oxalic acid is typically marketed as a wood bleach in hardware stores. It costs less than 10 bucks. All you need to do is mix a few ounces of Oxalic acid in a large tub of water, and soak the parts for 12-36 hours. You simply wipe the parts free of the rust occasionally with a white (soft, non-abrasive) scotch-brite pad, and you are done.
Rinse the parts off, and coat the insides of the tubing with LPS-3 rust proofing. Done. On to polishing...
The pitting will never go away, but you have not added any steel wool scratches, or scotch brite scratches, or any other damage. In fact, it will often leave the decals in tact! Especially if you cover them with tape first!"
The only place I divert from this process is that I treat the inside of the frame with framesaver. I did not tape over any decals, and they were not affected, but your experience could vary...