You can go thicker with bonding but you will still need a skim of thistle finish to get it smooth, or you could just finish with your usual filler

I have to do a lot of this kind of thing, best way is to use a good powdered filler (not that readymix goo).  I find Tetrion to be the best.

Plaster used as a filler is not a good idea, especially for thin bits.  It can't grip properly and as Jim says you need to Finish any backing coat so it's a time wasting way of doing it.

I buy Tetrion in 25kg Trade Packs but inside this pack are dozens of small packs, you only open one or two per repair, this keeps the rest fresh.

I get it from Grant & Stone builders merchants in High Wycombe, they have a decorators shop and that's where I get it.

Any good 'trade' decorators shop will keep this.