A: You need to visit the manufacturer’s website
A: “ A lighter weight Fast Set, affording the same volume of the 25 lb. bag in an 18 lb. package.” - a quote from Westpac
A: No, interior only. Mike
A: In would use at least two coats of all purpose drywall compound. Sand touch up and reapply where necessary. Doesn’t stick well to gloss
A: It will work, without a primer.... EXCEPT you'd have to work super, super fast. I suggest you get 90 minute mud and use this 20min version for patches.