I use Panels, in my case thin plywood of either luan or sande plywood, 1/8" thick which are glued to 1 inch by 2 inch (3/4 inch by 1 1/2 inch actual) pine backing boards, The backing boards keep the plywood panels from warping and provide some protection for the edge of the painting.

There are several reasons I use this as my painting foundation :

The plywood is relatively strong, rigid and more permanent than canvas or linen. It may warp but only a very tiny bit. It will never need re-stretching, will not rot or be attacked by insects.

Canvas or linen will continually stretch over the years. That's what happens to a fabric that is tightly fastened to a wood frame. If you look at really old paintings or even some newer paintings on stretched fabric, you will see the painting "sags" from the corners. Re-stretching a canvas or linen painting is best done by a professional and it is not cheap.

Plywood provides a stable painting surface. What I mean here is that when I apply a paint brush to the plywood, the plywood does not "give". Canvas, no matter how tightly stretched, gives when a paint brush is applied. This makes it hard to apply detail accurately.

Plywood surfaces can be sanded and primed to my exact requirement. No matter how it is prepared, canvas or linen will always have a fabric "pattern". This can be difficult to work with when doing detailed work.

Plywood on the supporting frame does not need to be framed. When I do a trompe L'oeil, I don't want to add a frame since I have usually already added a frame within the painting itself. Some people prefer unframed pieces of art. Framing is expensive. But at the same time, my paintings on panel can be framed if desired.