Hi 350ztn
Welcome to the Pool
I had a look around for other "Mixes" of spackle skim paste(new word for me) but with no luck

The secret I think is what lies behind the plaster is it a "cold wall"(facing outside), has the room been damp or in a cold area of the house?
Why I say this is under certain conditions the Skim goes brittle (too hot shrinks fast

) or doesn't dry(too cold)? or applied too thick(happens)!

Same is true when using filler with Body repairs(vehicles) you only have a narrow temperature range to get it "keyed in"
Best place to get advice would be either a Builders Merchants (Trade) or one of the DIY superstores where they have leaflets by the checkouts or ask the person at the customer service desk?
I found this link which may be of help with the job in hand?
http://doityourself.com/patch/properfillers.htm The others people here are very good at sorting things like this out.Wait to see if they have any better ideas than me?

As for smoothing the surface down wrap a piece of
medium grade sandpaper around a wood block should do a good job Doing it without the wood block will bring your hand up in blisters!

And you cannot get an "even finish" with this method(Makes more holes!

)
Ps "Sugar Soap" is good but is Caustic

(Have to wear gloves!)