I found that some of my "old" scullmounted animals have been slightly yellow over the years. I guess the explanation for this is that I haven`t removed the fat properly.
Is it possible to make them look as new? I thought I might try to bleach them again. But maybe boil them a little bit first.
Any recommodations? Have you done this before? Any tips/experience is appreciated!

I would NOT bleach or boil them. If you can find it, purchase a 30% or 40% proxide and mix it with a powder called Whitetning (I think). It's the stuff the gals bleach their hair with. Here in the states, we have beauty supply stores that carry it.
You can also find it at some of the taxidermy supply companies.
You mix it to make a paste and brush it on the skull and let it set. Wash it off and reapply if needed. Works like a champ!

i think what your looking for is "Clairol" you want pure white in about 30 strengh, and mix it with the "Basic" white power. make a paste and apply it to the shull, when dry brush it or rinse it off. don't get it on your hands, use gloves. this product can be found in a beauty supply, try to stay away from Taxidermy supply companys, they charge alot more.