I've got to do a bit of patching on a wall where I removed a cupboard.
Previously I've used plaster undercoat but having had some plastering
done
a few months ago I notice they used a sand and cement mix rather than
the browning stuff as it is less affected by damp.

What is the correct mix of sand and cement for undercoat when
plastering?

In article ,
" writes:
I've got to do a bit of patching on a wall where I removed a cupboard.
Previously I've used plaster undercoat but having had some plastering
done
a few months ago I notice they used a sand and cement mix rather than
the browning stuff as it is less affected by damp.

What is the correct mix of sand and cement for undercoat when
plastering?

If I mix it myself, I do 1:1:6 cement:lime:sand.
I did a load over Easter for a friend, and I couldn't be bothered
to mix it myself, so I just bought 4 bags of ready-mixed mortar.
I would guess this is 1:4 or 1:5 cement:sand, but it didn't say.
Worked fine.

If you want it waterproof, buy a mortar waterproofer to add to
the mix (or better still, a combined waterproofer/plasticiser).
This works very well -- I've rendered damp walls with it and you
can use standard gypsom finish coat plaster on top without it
getting damp at all. If you are doing multiple coats, only put
the waterproofer in one of the coats or it may have problems
drying out. Wait 24-48 hours between coats and before applying
finish coat plaster (mortar wants to be initial set but still
damp). Note that mortar takes weeks to reach final set, and the
surface (if you are leaving the mortar exposed) is easily marked
during this period.

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

Andrew Gabriel wrote:

damp). Note that mortar takes weeks to reach final set, and the
surface (if you are leaving the mortar exposed) is easily marked
during this period.

and if you are not leaving it exposed, then scratching it up a bit
before it sets can help provide a key for the skim coat...

--
Cheers,

John.

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In article ,
John Rumm writes:
Andrew Gabriel wrote:

damp). Note that mortar takes weeks to reach final set, and the
surface (if you are leaving the mortar exposed) is easily marked
during this period.

and if you are not leaving it exposed, then scratching it up a bit
before it sets can help provide a key for the skim coat...

Yes. Do this for all coats except the final one. Do it before it
reaches initial set.

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