External Nap Plaster

jobber

I am currently re-plastering a new build resulting from a very poor plastering job. I would appreciate comments on possible causes of the disaster.

Not long after completion cracks became noticeable particularly after a shower of rain. Most of the finished plaster had the appearance of a spiders web of lines spreading across large areas of theplaster. In other areas long straight lines, mostly vertical, could also be seen. On closer inspection a considerable area of theplaster was boast or hollow sounding when tapped & a milky white residue can be clearly seen along some cracks.

The render mix used consisted of:- sand, cement, lime and evo plast but I cannot say what the ratio was. The walls were scudded, one scratch coat applied & one finish coat applied.

Having stripped the walls it is now evident that the plaster in some areas has separated between the two coats and other considerably large areas have easily come away from the block with no traces of scudding left behind. It looks like the scratch coat was combed with a trowel which left 1/4" lines in it before the finish coat was applied.

Carlow52

The chaps at http://www.askaboutmoney.com/showthread.php?p=720472

seems to have covered most of the bases here.:)

The evo plast may in fact have been washing up liquid in an old container; have seen it done, with similar problems

mickdw

This happened to my house when being built.
The only cause I could come up with was over plasticised mix.
The plasterer used normal quantities of evoplast and then added a full measure of concentrate plasticiser in the same mix "to make it good and tacky".
It seems he had just discovered the concentrate stuff and was using it at will.
Needless to say he had to do the work all over again.

RKQ

I've seen a similar problem with "white" sand / cement. When applied on an elevation in strong sun light, it seemed to dry out too quickly and come away from the wall.
It was easily hacked off and had to be completely redone.