I have a hippo skull with the teeth in it. I've had it since 2009 (shot in 2008) and have a few problems.

1. It still smells a little like grease and seems to have turned yellow in places. What do I do? boil it more? It's too big for any pot I have. Get the taxidermist to boil it?

2. One problem with boiling: Part of it is plastered in to hide bullet holes.

3. I can't get the teeth out. the two big ones are stuck partway in and partway out. should I whack 'em with a rubber mallet or what? Is it OK to boil the skull with the teeth in it?

How about one of those taxidermists that has those dermstiead beetles that clean bone?

It sounds to me like your skull wasn't degreased properly in the first place. Boiling it more isn't likely to help and is more likely to increase the problem.

Dermisted beetles won't do a thing to help with the yellowing problem as the meat is off the skull already. Dermisted beetles only remove tissue, and it should be fairly fresh (or recently frozen). They won't touch old or rotten meat either.

I know when I cleaned my black bear skulls, they were pretty yellow with grease. In order to get rid of the grease, I put the skull in an old turkey roaster and covered it with acetone. I let it soak for a couple days, then removed it and set it in the sun to dry. I then followed it up with an application of hair whitening peroxide. Now the skulls are nice and bright.

You might want to try something similar with your hippo skull, although you're going to have to get creative on a container to hold the skull...

As to the tooth problem, I don't know what to tell you there. With all the skulls I've done in the past, keeping the teeth IN the skull has always been a concern.

Coleman fuel is easier than acetone. Tried to whiten a kudu skull (like Shof describes). Think the stain was iron and wouldn't come out. But, the Coleman fuel works on defatting the bone. BTW dermestid beetles eat away protein only.

I grew up working in my father's Taxidermy Studio, and recall that he used "White Gas" to de-grease many mounts including old bird mounts that had years of accumulated cigarette smoke and cooking / kitchen grease. It worked well even on badly stained & discolored white mounts like Mountain Goats, or the white feathers on some duck mounts. I'm not sure about the availability of "White Gas", so I appreciate the earlier posts suggesting Acetone, or Coleman fuel and will try that on some greasy Warthog skulls.
As to the stubborn tusks, here's an idea - carefully pour some liquid soap in the skull cavity around the tusk (to lubricate) but avoid spilling soap on the portion of the tusk you can grip, and then wearing rubber gloves to increase grip (latex dish washing gloves work well for this) and see if you can't manage to pull them free. Enlist the help of a husky guy with a strong grip if needed. the soap will not stain the skull and can always be washed away with water afterwards. I've used this technique to remove stubborn lower jaw Warthog tusks, and pulling stuck horns on trophies like Kudu, and Nyala. I'd be interested to know if this technique works out.

Whacking with a rubber mallet will likely break the teeth. The "plastered areas" will have to be redone (should not be plaster in the first place).

Boiling skulls can weaken and damage the bone.
Skulls should never be boiled. They are placed in a pot with something like a raised grate on the bottom to keep them off the direct heat. The water is brought to a boil and then the temp is lowered to a simmer. This "cooking" is done to remove meat more than anything.

A hippo skull would take a huge volume of Coleman fuel (which is the same thing as white gas) or acetone. I have used that method on smaller skulls like baboon with good results.
It would need a lid as it will evaporate quickly and using a plastic tub may not be a good idea with these strong solvents.

A slow method but one that works pretty well and will not damage the bone in any way is to soak in water with dawn dish soap. I use the Dawn Plus in the clear color.

Get a large plastic storage container that will hold the skull. Cover with hot water and plenty of Dawn dish soap. I would check on the teeth with the hope that a short soak in the hot water would loosen them enough to pull out. If they do, remove them and continue soaking the skull.

You can leave the skull in there for as long as needed. May take weeks or longer. If the water starts to stink, dump it out, wash off the skull and repeat with fresh hot water and more dish soap. Take it out and rinse with clean water and let it dry. You will know when it is dry if it needs to soak longer. This method works but it takes a long time.

On certain areas of the jaw, grease may be trapped in the bone with no way out. On these spots you may have to drill small holes that can repaired later. This will allow for the soapy water to help remove the grease.

When the skull is clean, whiten with a peroxide paste available from taxidermy supply companies (follow the directions and warnings). Rinse, dry and seal the skull with 50/50 Elmers glue and water or spray on matte finish.