Do i still need to mist coat with zinsser Permawhite ?

Redecorating our bathroom ceiling .Was going to just scrape off loose bits of paint where there were a few ceiling cracks but the paint was coming off in big pieces so ended up stripping all the paint on the ceiling to back bare plaster.I'm going to use permawhite satin on the ceiling as we had mould on there before even though we have an extractor.Do i still need to mist coat when using permawhite or can i apply it straight to bare plaster.If i do need to mist coat i take it one coat with 20 % water will do ?
Cheers.