Flat Plaster

How flat is flat in terms of plaster?
I'm not a plaster but I often have need of a good one on renovations, so far I haven't had much luck. It's been suggested I'm being too critical and expecting too much.
If I put a straight edge on a newly skimmed plasterboard wall should I be able to see variations across the surface. Even worse is if I shine a light across the wall, I can see ripples and marks everywhere. Is it me?
I have yet to see anyone use a "Derby" and one asked me what that was!
I would very much appreciate your opinion on this.
Thanks, Chas.

Hi Chas

Skimming will follow what is there - if the plasterboard is undulating the skim will follow this, as it is a smoothing off coat rather than a flattening coat. If the wall needs to be perfectly flat then a backing coat needs to be applied to plasterboard first.
Whilst undulations are acceptable as they are as a result of the skim coat following the plasterboard - there should be no acute or smaller bumps or hollows.

However shining a light across a wall is a test that many walls will not pass. When I am plastering - if I know there is going to be lights positioned anywhere on the plaster surface - I will make an extra special effort to ensure everything is good - but on all other walls they will look perfect under normal lighting conditions but may show a few blemishes when a light is shone across. Therefore whilst this is a harsh test that may reveal a few blemishes a degree of allowance needs to be given.

It is very hard to give you an idea of the acceptable allowance so the mark of good plastering could be that it looks good under the lighting conditions that it will be subjected to. For example wherever the light source is - even if from a window or door a good plasterer will always make an extra special effort in these areas. This is obvious to a plasterer and he should be doing this - but what is not obvious - if you intend to have lights shining across a surface then inform him so that he can ensure all is good.


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