Cleaning taxidermy and mounts

[Rembrandt]

Have collected about 40+ mounts over the years, some of the whitetail neck (white patches) have yellowed, it's not cigarette smoke, just age...any suggestions on how to freshen them up to look like new again?

 

[Shovelbuck]

I've had good success with a product I purchased from VanDykes called, Seybright. It works for me.