Flatness vs smoothness
You talk alot about flatness first then smoothness, ie its important to get it flat but not that important to get it smooth in the earlier stages. However, what's the difference, surly a flat wall is smooth and a smooth wall I flat?
I think I understand what you are getting at, but was just after a proper distinction between the two
A wall can be flat but if it is not worked in the later stages can remain coarse.
If the plaster is worked in the later stages the particles on the very top surface are consolidated and compressed to give a smoothness.
A wall can be smooth but not flat.
It can be undulating - but to achieve this a small trowel needs to be used. The more undulating the surface the smaller the trowel needed to fit into any dips in order to smooth.
If the surface is not totally flat the trowel will only be making contact with the high areas - the low areas will remain rough due to the trowel not making contact with them and therefore a uniform smooth surface will not be achieved.
If a wall is undulating the smaller the trowel - the more contact with the surface and therefore the smoother it will be.
The bigger the trowel the more important to keep it flat to stand greater chance of a smooth surface. When the surface is flat and the trowel is in contact with every part of the wall it will smooth all areas. When the surface is not totally flat - the trowel will only make contact with the high areas. this will result in the low areas not being smoothed out.
For example if on the edge of a wall your trowel is travelling in a sideways direction to fill right into the corner it has a chance of being curved (convex or concave). Then, in the later stages, if your trowel is travelling in an up and down direction it will only be making contact with the highest points either the hump of the convex curve or the edges of a concave - leaving the middle rough.
Why Flatness before smoothness?
The plaster can only be smoothed when it starts to firm up but also needs to have some degree of flatness depending upon the length of your trowel . Therefore in the early stages concentrating on flatness is vital to ensure uniform smoothness.
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