Only once I tried a canvas and didn't take to it really. So I tried making wooden panels, Egg Tempera fashion, heating up glue size Gesso for the undercoat. Nowadays Acrylic Gesso is much easier and maybe better. Made to measure 3mm-4mm MDF panels are more rigid than canvas for Acrylics. MDF made of dry fibres isn't supposed to be so prone to mildew, unlike hardboard made of wet ones. But best to seal the back first with a thin coat of Gesso. Then sand down the front and brush 6 coats of White Acrylic Gesso, each coat laid back using a 2 - 4 inch short hair varnishing brush, sanding last two coats with wet and dry paper. It makes a tough smooth brilliant white undercoat. When the painting is lit properly the Gesso should reflect back the light through translucent colours and brighten them.
- that's the theory, anyway!