rendering of concrete ceiling entrance

Sprayer would be alot more cost effective and faster than by hand,again a coat of sbr will help with bonding

You cant use a render mix because its to much weight on the ceiling so you should use a thistle bond mix with plenty of pva before so it bonds to the ceiling thats the best way

If its a concrete ceiling why are you rendering it if inside surely you could just PVA and skim it?

You can applierender to cement cieling by hand but first you need to achieve a grip on the surface for materials to adhere to if smooth you need give it a extra strength to achieve your desired result wire mess E.M.LSTARTING POINT BUT MY ADVISE IS GET A PROFESSINAL PLASTERER to address the issue correctly