"Andrew Gabriel" wrote in message
...
In article ,
"DIYer" writes:

"Andrew Gabriel" wrote in message
...
In article ,
"DIYer" writes:
I've just blocked up my garage door and plan to render tomorrow. It's
only
a small panel, 2.2 x 2.1m with a 1.0 x 1.2m window and I'm going to use
a
sand / cement mix with plasticiser.

I haven't rendered before and would welcome any advice or tips

What are you rendering on to?


7N blockwork

Not sure what the surface of those is.

Liberally slap on well diluted PVA (1:5) and let it soak in.
If it completely vanishes in seconds, wait for it to dry, and
repeat (second coat can be less diluted). In response to the
suggestion to wet with a hose - you don't want the blockwork
soaking wet as it will have no suction and the render won't
stick. Also, some concrete blocks (thermal blocks perticularly)
expand and contract slightly when wetted, and this can cause
the bond with the render to break if the blocks shrink after
the render sets.

Apply scratch (under-) coat render before PVA dries (at least,
start before it dries). Apply max 10mm thick. Rule the surface
off flat with a piece of straight edged timber, using a sawing
motion to cut away peaks. Fill in any troughs the straight edge
didn't contact and repeat. The whole surface should be left
rough, but flat. If it's too smooth, scratch over it with a
comb made from some nails poking out of a piece of wood or
hammered through an old float (called a devil float). The
purpose of the scratch coat is to leave a flat surface for the
finish coat, but with a rough finish so the finish coat can
key onto it.

Finish coat needs to be applied between 24 and 48 hours later,
when scratch coat is part set but still wet. Lightly brush off
any surface lumps (_lightly_ being important - at this point you
can brush off all the render if you keep going). Finish coat is
normally much thinner - thick enough to ensure that it completely
covers the rough peaks by a few mm, but doesn't need to be any
thicker as the scratch coat should have left a flat surface,
and if you make the finish coat thick, you risk undoing that
flat surface. Depending on the finish you want, you can polish
the surface, but you may have to wait for it to set a bit first.
The mistake people often make with both plaster and render is
to try poilishing it too much too soon, and end up making it
worse. At least with render, it sets slower, so you have more
leaway. The polishing is done with the straight metal edge of
the rendering trowel, dampened. Do not use a long straight
edge (derby) or your wooden straight edge, and never apply
the face of the trowel to the render, only the edge.

Protect from frost and rain for as long as possible. Render
takes about 6 weeks to reach full strength. Avoid rubbing the
surface before then, as you'll mark it.

I would use a 1:4* cement:sand mix. Scratch coat must be sharp
sand. Finish coat should be too, but you might get away with
building sand for an easier finish but possibly shorter life.
You should include plasticiser for ease of use, and can include
waterproofer. (Strickly, the waterproofer should only be in one
of the coats, and it depends if your doing in winter of summer
which one that is, but I can't remember which way round, and
it probably doesn't matter on a garage.)

*I've had a couple of builders mention to me that cement has
got harder over the years, and you can go with weaker mixes
nowadays.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]


Andrew

Thank you for taking the time to explain things. I've used building sand,
which here in Cornwall is derived from granite and comes more gritty anyway.
I've applied the scratch coat and am now waiting for the set to float it off
flat, as a trial for the finishing coat tomorrow.

In the meantime, I'm slurry tanking the single block skin garage walls