As far as letting these things sit in the sun, there is no need for it and a complete waste of time. You also need to mix your 40 vol creme with Basic White. All this stuff can be bought off of ebay. After applying a liberal coat of your bleaching agent, you want to wrap your skull in saran wrap and then place the skull in front of a heat source. A very hot heat source, but be very, very careful to not catch your wrap on fire. Rotate skull several times for about 30 minutes. Remove wrap and then dunk skull in super hot water. Rinse very well and if you have an air hose "boil" the skull under water with the air. Be sure to get all the crevices, with the air while the skull is under water.
And further more never ever boil a skull in the pot while removing the meat. Simmer is the key word.