Sometime ago I used to paint on museum board which I could purchase in large sheets from the art supplies store. I did this because I prefered the smoothness of the surface to that of the weave in canvas.
However museum board is thin and fragile and created some problems in applying gesso which could cause it to bubble and also in framing it is rather flimsy and requires a strong backing for support. My framer at the time told me many artist were painting on mdf board and so introduced me to this as a replacement for museum board and canvas.
MDF (medium density fibre) board can be purchased from the hardware store in large inexpensive sheets which make it economical. However it does require some preparation which I will briefly outline below and also in the attached video.
The board must first be protected from moisture and corrosive elements (especially if you use oil paints) by coating it with primer. Any primer should do but you can buy a specific mdf primer from the hardware store. I coat all sides and edges of the board with two coats on the back. I put only one coat on the side to be painted on because once dry I apply three individual coats of gesso.
This gives a good covering layer to the board after which you should not be able to see the original surface below. If you can still see the original board then apply a fourth coat.
Gesso, I have found, is quite abrasive when dry and you will find that your paint brush bristles are quickly abraded away to stumps so it is lastly important to give the whole surface a sand with a very fine sandpaper. You don't have to press too hard and undo all your preparation, a light sanding is fine and by running your hands over the surface you'll soon find that it doesn't take much to smooth it off.
MDF board can also be trimmed to fit into any spare frames you have though make sure you trim the board nice and straight. Once again prime the exposed edges after you do this.
Lastly the sheets, while coming in different sizes also come in different thicknesses as well, ranging from about 6mm to 18mm, and the thicker the sheet the heavier it will be. I usually choose a 6-12mm range, anything thicker starts to become very heavy if you use the whole sheet and then add a frame to that.
Tip: With each coat of primer and gesso rotate the board 90 degrees and paint perpendicular to the layer over which you are covering to counteract the emergence of deep micro channels in the final surface.