In article ,
littlehobo writes:

Hi,

I plastered my ceiling a couple of months ago and was really pleased
with the finish. However, I wasn't sure of how to correctly finish
with the float, and ended up finishing with a wet sponge/brush. I now
have to do a wall, and really want the smooth finish from the float.
Any tips? Do you use it in the same way as a trowel, or do you put it
flat onto the plaster, in which case, how do you prevent suction?

Let's start by making sure we're talking about the same tools.
A float is a wooden (or nowadays, often plastic) faced tool,
usually rectangular. A plastering trowel is similar but has a
steel face, and importantly, two long straight polished edges.

Floats are used for rendering, screeding, and occasionally for
scratch (base) coat plaster. They are never used for finish
(top) coat plaster. Trowels are used for all these too, but
also for finish coat plastering.

So to answer your question, you polish off with a trowel.
I've posted the details before, but I'll run through them again.
Firstly, a new trowel is a swine to use -- it will have sharp
edges and very sharp corners which even a professional plasterer
will dislike, and a beginner will find very difficult to use.
Traditionally, a plaster would give a new trowel to a renederer
for a few weeks to get the tool broken in on sand and cement,
i.e. edges polished up and sharp corners taken off, and also
it gets the spring in the steel working nicely, which it doesn't
from day 1. You can polish up the edges on a flat faced grinding
stone or the flat face of a house brick (easier to find on a
building site) by using a sawing action, holding the trowel edge
at an angle against the flat face of the brick. This is also
useful to repair a trowel if you get a nick in one edge. Drag
the corners across the brick face to take off the sharpness.
In use, they'll wear to become rounded, which makes the trowel
easier to use. You might find digging through [grand-]parents
old tools to find an old trowel much better than buying a new
one, provided the edges aren't damaged.

As for the plastering, to start with you just get concentrate
on getting what you mixed up onto the wall with complete coverage
of the area you are doing with that load. Ignore ridges, lines,
etc, as there's nothing you can do about them yet, other than
move them around which is not useful. Having got all the plaster
on the wall, make any adjustments to thin out any excess areas
and fill in any deficient areas. Now you have to wait for the
plaster to start going off a little. When it has, you will find
you can go over the area and remove the ridges created by the
corners of the trowel. You'll create an equal number of new ones,
but they'll be a bit smaller (if they aren't, stop and wait for
the plaster to go off some more). You basically keep repeating
this process until the ridges are gone. What you must do is stop
working an area of plaster when you aren't improving it -- it
needs to go off some more and you will only damage it by working
it excessively before it's ready. The biggest mistake people
make is not leaving it alone to go off some more. If you are
working a big enough area, then going over it cyclically might
give each bit enough time to go off some more before you get
round to it again. As the plaster goes off, you will have to
press harder to have an effect on it, and you'll need to
lubricate the surface with water, traditionally flicked on with
a large brush, but I find a plant sprayer easier to use, not
having years of practice aim with the brush. This action becomes
the polishing process as the plaster becomes nearly set; the
smooth trowel edge takes off the peaks of the plaster surface,
carries them in the lubrication water, and drops them in the
troughs. However, don't get too carried away -- you can polish
up the plaster surface so it feels like a sheet of glass and its
horribly tempting to do so once you get the hang of it, but
you'll find you can't paint it afterwards. If you do go too far,
you'll have to sand the surface and/or excessively dilute the
first coat of paint, or you'll find the first coat comes off
when you try to do the second coat.

As you already commented, you don't ever allow the trowel to
go flat onto the plaster or it will stick, and you'll be very
lucky to get it off without pulling the plaster off the wall
too. (You'll probably do it accidently a few times, and have
to repair the damage;-) The face of the trowel is only used
to carry the plaster to the wall initially.

--
Andrew Gabriel