Hi again, Victoria!

Sound like you've got yourself a perfect wall!
Tar paper is a regular supply or the masonry store and home depot carries it as well. You will have a hard time fastening the tar paper and wire mash to the wall and I would advise to con tact a professional mason/plasterer to do it.

Theoretically you can "go" directly on to the wall without paper and mesh (based on what you say it looks like it is structurally sound, clean and dry and has no cracks). However in such case you will have to sandblast it clear from paint and "roughen" the surface with hammer and chisel (give the wall a tooth) to make the new plaster coats stick to it well. Use of the professional plasterer to do this is essential! If done improperly fresco will fall of the wall in no time.

I think you should go with tar paper + mesh, scratch coat, rough coat, float/brown coat (arriccio), intonaco. Use the plasterer to lay tar paper, fasten the mesh and build first two coats at least.

Post some pictures of your work in this forum as you go along.

Ilia

PS. Do not hesitate to ask more questions - i am happy to help. Yoram Neder must have a lot to say about outdoor fresco, I hope he reads this post.