If you plan to use the salt dough ornaments for the holidays and then toss them, there's really no need to seal. If, on the hand you'd like to keep them around for years, I'd suggest spraying them with a matte clear top coat spray paint, like Rust-Oleum Matte Clear. Clear-coating your ornaments is also a great idea if you've painted them to protect the painted details. Using Mod Podge (whichever gloss level you prefer) to seal the salt dough ornaments is also quick and easy.