Hardboard / Masonite
Masonite boards work as great practice pieces or can be used to hold your watercolour paper with a firm surface behind it. They are inexpensive and unprimed. It is 1/8" in thickness
Cradled Birch Boards are made of raw birchwood, these panels are super smooth and finely sanded. They are built with either standard depth (1 inch) or gallery depth (1.5 inches), making them perfect surfaces for resin and acrylic painting, as you won't have to worry about sagging or warping.
Birch Panels: Appearance-wise, these have pleasing grain patterns and are a nice, uniform light colour. To make these panels as strong as possible so they'll resist warping and other moisture damage, the thin layers of wood that make up each panel are stacked so their grains run perpendicular to each other.