I'm having an extension plastered with sand and cement render then a gypsum plaster skim. The plasterer was meant to be doing the job in 3 days, applying a scratch coat first day, then another coat of render second day then skim the third day.

He has just rung me saying he is on another job and can only come for two days so he will be using rapid set cement and doing both render coats in one day. I don't like the sound of this and I am worried about the render dropping off.

Any advice? I have already bought the cement (normal Blue Circle stuff in the brown and blue bag) which I thought was the right stuff for the job.

Sack him. Get someone who does lime or mud rendering. Or do it yourself.

I would also go for a cement/lime mix , although there is rarely a problem with [internal] cement rich mixes provided the background itself is strong itself ie. concrete block.

I don't agree about ease of application as a lime mix is easier to apply in my experience.

I was taught to use 3 and 1 sand/cement for most internal work as an improver plasterer in the early eighties. The reason for this was that the work involved damproofing old houses, and I was taught [wrongly] that a rich impermeable mix was required to remedy rising damp. I used this mix internally for years, though always applying the golden rule funcrusher mentions about subsequent coats. Lime mixes were only used outside, although the idea of impermeable mixes stopping water penetration is a common misconception and can be disasterous for external walls, resulting in map cracking and 'blowing' of the render.

Having eventually learned the error of my ways, I now use 5:1:1 Sand:Cement:Lime for most [though not all] applications. A very weak background [wall] will require weaker mixes, as will any buildings of historic value,

How big is the extension Chris? most decent plasterers are able to render several rooms in a day. I would not use fast set cement for this job. The suction in the blockwork will mean that the render will be dry enough to float in no time at all. I don't think your plasterer knows what he is talking about.

Shame you're not in South Wales, that's no more than a mornings work. Did lafarge advise a sand/cement/lime mix?

interesting stuff guys.
I also have a rendering querie which i'd appreciate any opinions on:
The area in question is what was the outside of a gable wall on a 16th century Guernsey farm house, (the whole house is built of 1m (+/-) solid granite walls! and as they did in those days has a circa. 3degree lean from the base to the eaves. It's two storeys with a loft and L shaped in plan.)
We've built a large 'extension' off the end of it creating a sort of court yard and bringing what was the gable end inside the new dining room, the FFL of which is also now circa 500m below that of the old house. Therefore i need to tank at least 650 mm up the wall, i intend to use a 10 spike osmosis system with a tanking render on both ground and wall, followed by a flexi mebrane at the junction. The main wall however is to be plummed up and plastered, we can't stud it out as there's no space so were thinking of cleaning up the granite and using a lime mortar to roughly re-point/fill holes then using one thick coat of easy bond, followed by a 3:1 sand and cement render with fibres and water proofing agent @ 10-12 mm with a scratch finish, then a 4:1 mix with some plaz, and finally a scim of gypsum...
We've done all the rest of the house in lime mortar and I can't help thinking that there's a better method than the one we've come up with to render this wall, especially with the moisture that will inevitably be in the wall from 300 years of rain and not forgetting the fact that the other side of this wall is a huge fireplace which in winter will agian be in use!
sorry about the essay, any ideas anyone?

Chris, may be he isn't a proper plasterer. It is easy as pie to get two coats on in a day! one in the morning one in the afternoon. Scratch it the next morning and set it. ( do another job in the afternoon)

Why do you think it will be damp? where is the water table level? how can water get through granite?

I think all you need to do is lime plaster it -- no damp proofing no membranes no problems.

My reasoning is that the water vapour pressure in the air in the house will be higher than that of the air behind and in the wall in question so a little moisture will pass out of the room into the ground! Not the other way round as long as there is no running water then no problems and if there is running water then you will be fighting an expensive loosing battle trying to stop it.

Yes Fun, you are right to pick me up and hang me out to dry but in my favour are local granite and no mention of damp in the house ( has this to with lime?)

The idea of 300 years of rain is not relevant as generally under houses the ground is bone dry -- 300 yrs of no rain!

What is the house built on topsoil? rock? or what -- where is the water table?

You will be OK then on all counts.

I've just plastered blockwork (parge coat) myself, it took about three days but I was on my own and never done this before; plus the blockwork and cement joints were all over the place. I believe any base coat can fairly be easily be done by a DIYer its the final skim coat that is the tricky bit to get that polished finished. I used 2:1 NHL 3.5 to plastering sand. I've seen a few sites describe how to first batten the blockwork with wood to the thickness of the base plaster and the batten width to the size of a straight edge (spirit level). You then have a perfect edge to plaster to at the thickness that you want. Take the battens off when dry, or possibly still damp, then fill the whole as either side is now level.

If you do go for it yourself you'll be learning a new skill, can choose the mix you want and have a great sense of achievement. Only word of advice is it makes a hell of a mess and wear long-sleeves and latex gloves. I didn't wear a long-sleeve on the first day and now have some lime plaster burns that are slowly healing