If you want to plaster then you should use the proper materials. If you had plaster over wood lath then you need to use real gypsum plaster made for such an application. If you use USG then theirs is called Red Top Gypsum Plaster. If Gold Bond then the most common brand is Two-Way Hardwall. Mix these with sand according to the instructions on the bag. If what you had was veneer plaster which is about 1/8" thick over gypsum board then you should use veneer plaster materials. Check the web sites of National Gypsum Co. and USG for the right stuff. Mixing entirely different materials not meant for such applications will cause you lot more work than doing it with the right stuff. If you have any more questions about your specific conditions fire away.