For me printing directly onto a wall, especially outdoors, is the most exciting. The wall must be plastered with a fine plaster and the finish is best when it is smooth… and then it is be sealed. It is a good idea to put a hardener on it to help the emulsion take to the wall and make it more resistant. For outdoors I have a mobile darkroom with a window at one end which we fix against the wall around where the image is to be printed. The liquid silver gelatin I apply (in dark with red light). It takes about 2 hours to dry. I use a projector to expose the negative, followed by the standard darkroom process of dev, stop and fix. Washing is essential. To be on the safe side I use hypo followed by 45 minutes washing with a hose pipe.