Hydrogen peroxide (more concentrated the better) mixed with so clean detergent booster, make into a page and let sit on the skull for 1 hour, will take that crap off and leave it “bone” white…
Make sure to remove the ear canals. It can help sometimes to drill a small hole into the brain cavity from the bottom with a 1/2" wafer blade to flush everything out of there. On tough, greasy skulls, I sometimes let them soak in a room temp hydrogen peroxide/water solution for a day or two and it seems to help whiten them up without weakening the skull too much. Let us know if you find a way that works for you!
When I started doing euro mounts I liked the idea of burying them and letting nature do the work, but what I find is that the end result is nothing like the end result of using a tank and buying the correct beetles. I just did two mounts this year, one was burried for a length of time and one fresh harvest, skinned and cleaned by knife as good as possible before a slow boil for a few than a long simmer.
The one burried and allowed to be eaten on by bugs was more fragile and nowhere near as clean as expected. Nature also did a good number of the bone/skull as far as degrading it and making it more fragile.
It’s my opinion, but cleaning it as quickly as possible and boiling / simmering in a powdered detergent before intelligently using a pressure washer is the best. If you choose to bleach or peroxide it you can but I also finish with a few coats of a clear coat from a rattle can to keep it from drying out too much