Hi Celeste,
Hard wall plaster repairs are treated the same way as lathen plaster repairs.
http://www.plasterbrokers.com/restlath.html
If this does not help let me know.
P.S. Plaster of paris is not used for repairs. All types of plaster have different setting times and user characteristics, for specific uses. Plaster of paris when troweled will set very fast and peel if you try to work it as it sets.
The best plaster for patching is cornice adhesive. I has all the right characteristics to be user friendly. It will trowel smooth, fill quite lage gaps, scrapes back smooth without peeling while "green".
Green is the term used for plaster that is set but still wet.
Spakfillers etc will do the job ok for very small jobs, but it workes out very expesive to use if you have to buy more than one tub. For $22 you can buy a 20kg bag of cornice adhesive!
Cheers.