I think you plan to paint on canvas & attach it to patio wall. Is this a covered patio? Canvas & water don't mix too well, tend to stain if rain & dirt mix & hit it. If you want it to last better I would think painting a piece of 1/2 in plywood with undercoat(primer) & couple of top coats of white- could do all this with a roller. Then when that is dry do your painting on it. Canvas tears, gets holes punched in it when someone swings broom around or rake etc. I just don't think it's very practical. I've had to try to work water stains, smoke & dirt off of oil paintings & they don't clean up too well.In a year outside the canvas will start to degrade & may shred, tear or get holes in it & spiders would be in paradise to use back of canvas on stretcher frame for their breeding ground. The well prepared plywood covered with your design & then sprayed with clear matte or satin Krylon to completely seal it would last number of years & if screwed into wall wouldn't be much room behind it if any to produce tons of spiders. I would get the 2 oz. Patio paints at Michael's or Joanne's craft stores. Moisture, dirt & wind would not be friends of canvas. My grandmother's lake cottage had partly enclosed wrap around porch, at far end was a cot sized mattress on springs & wooden ends on this swing but the back was cream colored canvas & it had to be recovered every couple of years. The porch had full roof & screened areas that were covered with wood panels in harsh winter weather. I remember the canvas was often in bad repair but it was such fun swinging on the swing that was held up by chains to roof. It was at least 50 yrs old! Cottage had real block ice in the icebox & I thought that was so neat, that was where they cooled the cantaloupes & watermelons.

how about painting directly on the wall instead of canvas?

House paint is a great choice for any outdoor painting. You can buy base paint and "universal tint" (paint stores or amazon) to make your own colors, instead of buying some of every color you need.

Some paint companies will give you free tint--though the one place that used to do that here has stopped, so it might not be easy to find.

Painting directly on wall can spoil the paintings in rainy season and even winters. Use wooden thick canvas for painting. The frame is same and you have option of changing the paintings.