I have cleaned and boiled many skulls in the past few years, with no
problem. This year 2 of my bear skulls turned out porous looking.
I use Sal Soda, same as before. What did I do wrong, and can they be
made to look more smooth? Any help is appreciated.
Sounds to me like you boiled the skulls to long,sal soda will eat bone if boiled to long. If i remember correctly you should not boil more than a half hour.
Try mixing half water with half Elmer's glue, and coat the skulls in that. You can let it dry and repeat if necessary. This might give your skulls a more natural feel and look, and if you don't like it, you can just soak the skulls in water to remove it.
Good luck!