Mural painting on stucco. Need help !

I am designing a garden for the back yard of a client.

The house has a detatched garage, which is covered in stucco, like the rest of the house. One wall of the garage will provide a backdrop for a large portion of the garden. The wife of my client is a very good artist and we came up with the idea of painting a "Monet-esque" type of mural on the wall to make it blend in and not have it be an obvious wall.

The problem is the rough texture of the stucco. It's actually the "smoothest" type of stucco finish, but still too "rough" for the artist to create that "fading away" technique of the paints, and the process it takes to achieve that look.

Does anyone know what you can apply OVER stucco to smoothe it out and make it more "flat" and conducive to this idea? We don't need it to be perfectly flat, just not so "rough" as it is now.

The wall itself is in good condition...no cracks, or flaws. We know we'll probably have to sandblast it to remove years of layers of paint, but we're stuck on what to top coat it with, that won't require a lot of maintenance.

Has anyone ever done anything like this before?

We're trying to avoid knocking off the stucco, the scratch coat, etc.

Thanks,
JD