Skim coat with plaster of paris mixed with joint compound
Don’t mix use the correct product.
 Mixing can affect the strength and texture.
For a light skim coat use a topping mud or topping compound or add water to standard joint compound or mud. 

Prior to coating make sure to clean the surface, the leading cause of failure is surface contamination. Dust is a big one, smoke , oil all of these may be hard to see but can cause failure. I love TSP Trisodium phosphate, make sure to use gloves.  After cleaning a nice wide knife and topping mud I can skim a large room in no time.