Walls and Ceilings - spraying on spackel

Have to skim coat entire old construction room. In the past I have seen people skim coat/spray spackel onto walls,sand and spray again to get a truly smooth outstanding finish before painting. Never paid much attenion and there for this question. I have airless sprayer but dont know procedure or correct thinning mixture. Please help. Thank you, Bill

Any link to correct site also appreciated.

Welcome to the forum
I have never known of anyone spraying a coat of mud on the wall although I have seen finishers roll a thinned down coat of mud and then smooth it down with a wide knife. I assume what you saw sprayed on the walls was a orange peel texture [other textures are also sprayed] There is also a product by USG called 'first coat' and some 'high build' primers by other paint manufactures. These have to have a pump cabable of spraying at least .75 gpm to apply them. I used to know how big a pump needed to be to spray an orange peel texture but have forgotten long ago - however small airlesses won't do it.