Look for a rubberized paint to keep out more moisture. It’s around $10 a can. I’ve seen them in a rainbow of colors at either a Lowe’s or Home Depot. Any paint used will make the canvas hard and uncomfortable to sit on unless you add cushions.
Maybe using Rit Dye would be cheaper to cover any stains left by the mildew and it would retain the flexibility of the canvas. Spray with ScotchGuard to seal color in and water out.
They have special fabric paints in the craft section of you favorite store.
Go to your local craft store. Ask them about what to use and check the price. I bet it is cheaper to buy the stuff elsewhere. Our craft stores around here are so darned high.
to get the mildew off , i have used a solution of bleach and simple green and water 2parts water and one of each this will remove the mildew, if you can take them off the frames use rits dye
Simple Green and bleach with water will remove mildew and mole from anything and will not come back