Why has new plaster skim coat not stuck?

[rrh]

Not diy but I got a firm in in January to hack off, re-render (with waterproofing agent) and reskim one of my conservatory walls which used to have a problem with penetrating damp. The single brick course (no cavity) wall wasn't properly dry but the render seemed to dry off OK - eventually - so I thought it would all be OK. So did the company when I dragged them back six weeks on. By today the skim coat was dry enough to paint but I found it was blown in several places:

are blown patches elsewhere on the wall too apart from those shown.

Of course I'll get the company back Monday but does anyone have an idea what went wrong? Eg:

- Render actually didn't dry off properly at the outset (because of state of wall behind/continuing penetrating damp) - NB the patches of discolouration.

- It was done in that horrible cold spell and the conservatory is unheated.

- The nominally 3mm skim coat seems as thin as 1mm in places.

I've used the company before and they're OK but I'm not sure whether this is bad workmanship, bad luck or something else.

TIA

[simon beer]

eventually -

discolouration.

Had this happen to a room I had re plastered once. Not done by me. I think at the time we couldn't really work it out but thought that it may have been that the render coat had been finished off too smoothly, thus in places there wasn't enough of a key for the skim. Pain in the butt but it needs to come off. Give the walls a good going over with PVA and re skim, if you can mechanically make the render a bit rougher all well and good. Try scraping a bolster along it, digging the corner edge in. Don't worry about the fact that in places the skim was only 1mm and in others 2 or 3, quite normal I would say.

[rrh]

Thanks; will try this one on the company.