One coat render

[jeffersgb]

Builders rendered the rear wall of our house last October and weeks after I noticed fine cracks appearing in it with accompanying hollows.
I was expecting and wanting a nice smooth, flat and even surface which could be painted over but it looked as if it had been put on with butter knife. It was uneven and bumpy looking especially when the sun caught it at a sharp angle.
I complained and they returned in June to hack it all off down to the original brick finish and redo it.
This second time they did it with one thick coat - no scratch coat.
In one small area it has failed again (cracks and hollows) where the base was breeze block. Where it was brick it is solid. But again the top surface is terrible, worse than the first job.
I would be grateful if someone could answer the following questions:
1. Is it possible to get a smooth and flat render surface and what is the secret in achieving it?

http://uk.f255.mail.yahoo.com/y5/s/viewphoto;_ylc=X3oDMTRlaHM5Y2dsBEFjdGlvbgNUaHVtYm5haWwgY2xpY2tlZARJbnRsA3VrBExua1R5cANSZWd1bGFyBFJlc1Bvc0EDMARSZXNQb3NSAwRTcmNoQ3VycgNwaG90bwRTcmNoRGVzdANwaG90bwRfUXVlcnlJZAMxMDQ0MzAwNjE5NDRkYzhjOTQwOTIyYgRfUwMxNTA1MDA3NTY-?show=largephoto&folderid=%2540S%2540Search&mid=1_22_1_2544791_0_AJUKDNkAAYDwRNyMYwzUXD4d31Q&partid=2&name=Imported+Photos+00001.JPG&size=146432&search=1
2. Should rendering be done with one just coat?

Thanks[img]

 

[renderman]

I heard that you dont need two coats. One is just as good.That's what I did last weekend and it looks good.
Its smooth as well.Just took my time.
Surprised nobody else have answered you about this.
All the experts must be hiding something.
Sounds like you had cowboys doing yours.

 

[jbonding]

external rendering should never be done with one coat, it will blow off or crack then blow off. The scratch coat is not only for building up or getting a surface flat. your scratch coat should be a stronger mix than your second coat, what this does is give you more time to rub up on your second coat and reduces cracking. your second coat should be mixed weaker. What your sratch coat is doing is controling the suction for your top coat which reduces cracking,blowing off and gives you a chance to rub up and achieve a good finish.

 

[jeffersgb]

Thanks for the reply jbonding. I might have guessed as much. It'll all have to come off and be done for the third time.
But by someone else this time and I won't be paying.

 

[Builder.Bird]

Hi,

I always use 2 parts building sand, 2 parts plastering / washed sand and 1 part cement.

The first coat (scratch coat) should have retarder in it. This stops the mix from drying out too quickly and cracking.

The 2nd coat (top coat) should have waterproofer in it to reduce ingress of water.

The amount of times I have been called in to put other peoples work right....

 

[legs-akimbo]

Ayup J.B did you read that "WATERPROOFER" in the top coat eh !!! shock horror. Time for you to go into one me thinks.
I see plenty of one coat rendering, all that about control of suction bla bla bla is blowing hot air, if you prep correctly you can get away with one coat rendering albeit only on garden walls and the like. I do see alot of houses that have clearly only had one coat of render mind as the blockwork is clearly visible (grinning) ......personally I would never render without a scratch but alot of building contractors pay handsome money for boundary walls with a float finish for one coat renders and if thats what they wants, thats what they gets