Sally's Beauty Supply. 40 volume peroxide. Available in the stores or online and cheap. Get the gel kind. Brush it on, let it sit for a day or two, brush some more on and after a few days rinse off with a hose and air dry. Your skull will be whiter than white.
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Question 2 is there something you can coat the antlers in to keep them from whitening or fading?
IE I want the antlers to retain or even enhance their natural darker look...it is the white skull/dark horn contrast I love on our skull mounts as we have been fortunate to shoot some darker horn bucks. As the years go on though the antlers and skull seem to trend to the same tone...horns lightening up via sun bleaching, skull yellowing out some.
"Only takes a second to change your season" Dad, 1980
Buy a box of borax from Walmart and some peroxcide,Buy one of those throw away aluminun turkey pans. Mix borax with peroxcide into a paste lay it on thick! Wait about a hour,rinse.Not white enough do it again until your satisfied. For your atlers! use a little wood stain.
It sounds as though it may not have been degreased properly before whitening the first time. Before you try to whiten them again, I would suggest trying to get any remaining grease out of the skull. You can soak the skull in a container with hot water and dawn until the water does not cloud up any longer. It's best to keep the water at 115 degrees, but most people don't have the set up to do this. You can change the water everyday,as an option, until the water remains clear. The water won't hurt the antlers. After the water remains clear, take the skull out and let it dry. Then buy some 40 volume peroxide and basic white powder. (Don't use the 40 volume creme. It has oil in it.) Mix these together to form a paste and brush on skull. As a safety precaution, use gloves, safety glasses and a respirator. I wrap the skull in plastic and put it under heat lamps for about an hour, turning it a few times. Rinse in hot water and let dry for a few weeks. If any grease is remaining, it will be easier to see. If you notice more grease, repeat process. Once you are confident there is no longer any grease in the skull, you can spray it. I use Krylon matte finish. Or, you can take it to a taxidermist.