I simmer in dish soap for an hour on, pick crap off until carpel tunnel sets in, repeat until it’s pretty darn oil free. Pressure wash a bit to get brains and nasal cleared. Careful if you wanna retain the olfactory structure which is difficult without beetles.
Clean water simmer again with disinfectant like baking soda and maybe a cap of bleach to start whitening. One it’s spic n span I let dry and paint in a paste of hydrogen peroxide and baking soda or that hairdresser stuff 40 works great. Use old toothbrush inside and out to kill anything especially smells. My wife has an excellent sniffer.
Anyway, when I can’t get them really clean, I do what is pictured. This was a nice deadhead I found while pheasant hunting Palouse. I couldn’t get it perfect so I ended up painting the entire skull and then the kids helped me detail out. Got another couple I did like this they turn out good and the wife let’s them in the house. Totally sealed with coats of paint. Use tinfoil while simmering and never let bases touch water.