I submerge it. You can also mix it with Magnesium Carbonate and make the 27% & 30% liquid peroxide into a paste.
The 50 volume Hair developer is only 15% peroxide. 10 Volume starts at 3% for each 10 Volume you go up the peroxide volume increases by 3%. So 20 Volume would be 6%, 30 Volume would be 9%, etc, etc.
You have it right on wetting up the skull a bit before adding the hair developer but you don't want it to be soaked. Wrapping it up in plastic wrap and placing it in a warm room or by a heater (keep it rotated and not to close) will aid in the whitening.
If you paint it on and allow it to dry, once it is dry, it is done working unless you wet it back up.