Outdoor wall art suggestions.

Try housepaint

I'd go with a good exterior house paint. It will be waterproof but know that ultimately, NOTHING is sunproof. The next best thing to sunproof, however, is to spray your finished art with a UV protectant finish. Check the label, I think Rustoleum makes them in gloss and satin. That will add several seasons - but you may need to retouch - so remember what you use!

You should be able to use any paint made for outdoors and weather. I would top coat it with a clear marine top coat for it to last even longer, I would use a couple of coats.

This might be nice. I would think a shower curtain would hold up well to the elements and there are some beautiful artwork ones available.

https://www.hometalk.com/diy/decorate/walls/easy-large-wall-art-using-a-shower-curtain-7489290?r=2

My husband painted this mural in our yard using acrylic paints. Depends on how intricate you want yours to be and as others have said sun will do more damage than if it is in a shaded area like ours. Absolutely no finish needed. This is not an oil painting on canvas. The lower pic is the first part of this mural and is about 18 years old. (no top coat and we live in Maine. My husband is an artist)