The video below describes the layers of a painting quite well.
- Wood panels can be found many places. Furniture grade is a good indication of quality. Plywood is usable, but will absolutely need to be braced on the back to prevent warping. For plywood, Grade A is what you’re looking for and a tight grain like Baltic birch is good. The local Habitat for Humanity ReStore is a good place to look for wood cabinets and even hollow core doors that can be made into nice painting surfaces.
- Engineered wood products such as MDF (medium density fiberboard) or HDF (high density fiberboard). These are made from wood pulp that’s pressed and glued together to make sheets. Both are available at Lowe’s, Home Depot, etc., but only HDF tends to be available in 1/8″ and the 3/4″ MDF is quite heavy.
- ACM or aluminum composite material panels such as Dibond, Alumalite, and Maxmetal. These are thin sheets of aluminum that sandwich a polypropylene core. ACM is available at sign shops.
- Paper can be a great surface. You’ll want an acid-free paper, so no brown paper bags or newsprint. There are many papers made for watercolor or printmaking that are perfectly acceptable. The paper should be mounted to a rigid surface but that can be done after the painting is complete. Follow the same sizing/ground application as for canvas or hardboard.