DIYbanter - View Single Post - Internal rendering/Plastering with Sand/Cement/Lime Mix

(Andy Hide) wrote in message ...

I have previously used a mix of 5:1 Sand/Cement to patch up blown
areas of plaster in my c1910 house.

snip

I have now changed to a 6:1:1 Sand/Cement/Lime with the sand being
Wickes (red bag) "for use in bricklaying and internal rendering".

IMHO your plaster is too hard/brittle (assuming the wall - brick? - is
completely stable: does it move if someone walks on the floor above or
shuts a door hard?). Soften it by doubling the lime and halving the
cement content - eg 6:0.5:1.5 - keeping the 3:1 overall ratio, you
could possibly increase the amount of sand to an overall 4:1.

I could mix by spade.

Minor cracks in the background shouldn't maatter too much as the
finish coat will cover them up.

make sure you trowel over, preferably with a wooden trowel to get a
smooth sandy finish then rake over it to get a good scratch scrolling
finish to key for the finish plaster.

Your finish coat should ideally go on about 24-36 hours after the
background is put on. It's much harder to finish plaster onto a dry
sand/lime/cement background.

Is
this the same as plastering sand ? Would plastering sand be better in
this case ?

IMHE almost any building sand will do the job. That's unlikely to be
your problem.

Are you certain the wall is not vibrating or moving?

snip

A few thoughts on what I think may be going wrong:

1) I'm using hydrated lime which is white and very fine powder. Did I
get the right stuff from the BM ?

that's the right stuff

2) At first I thought this was the problem of the wall sucking out the
water from the render too quickly thus it drying and cracking - but
having throughly soaked the wall to the point where it wouldn't suck
in more water I don't think this is the problem. Perhaps it's too wet
now ?

not sure on effect of wetting he that may be a major part of your
problem.

The lime and to some extent the cement in the mix should make the mix
adhere to the wall by suction, so you don't normally want to wet the
wall. It may be the prior wetting is making the mix move on the wall
during the set & hence leaving cracks.

When you *do* wet the wall is when you finish plastering on to a
sand/lime/cement background that has been allowed to become too dry.

Try putting the background onto a dry wall.

HTH