My gallery includes paintings in various media (oil, acrylic, watercolor) and on various surfaces (paper, canvas, canvas board, slate). To provide physical and UV protection the arylics have been sealed with a water-based polycrylic coating. This coating also enhances the colors of the painting. The 2-tone oils also have a protective coating although I normally frame these behind glass to help keep the paper straight and to protect the image
While painting on slate, I like to use its natural texture and color as part of the painting. Getting the slate wet darkens the piece and brings out all the hidden colors which helps me "see" what to paint on it. When finished, these are sealed with polyacrylic to bring out the texture and colors of the stone and to protect the paint, which can come off the smooth surface if scratched. A 12x12 can be heavy, around 6 pounds, but can be hung on a stud or proped up, as on a mantle or shelf.
Pencil is also a media which I have done quite a bit of, most of which I have given away to family.