Rendering S&C problem

[tireetim]

I'm trying to learn how to S&C render, using a brick wall for testing. I am using S&C in 4:1 ratio with added 3 in one additive to slow the setting time down a bit. Sand is a yellow building sand that the builders yard tell me is the correct one for rendering (I asked them because they don't have any bags specifically labelled as render sand). My problem is that when I use a plastic float to help even out trowel marks and give a bit of a textured finish the render doesn't seem to set very hard. Running your finger over it seems to crumble from the wall. If I don't float it then it sets rock hard. I don't think it was too hot or dried too quickly so I can't understand why the floated bits just crumble? Anybody got any ideas?
Thanks.

 

[Alastairreid]

//www.diynot.com/forums/viewtopic.php?t=142247

 

[tireetim]

Ah yes, thanks Alistair. Perhaps I didn't float it enough. Was following a youtube video that said leave it a couple of hours until it had started to firm up then float it in a figure of 8 pattern. I didn't want to do it too much as it seemed to roughen up the surface too much (made it very sandy). Didn't bother with a sponge as I thought this was optional and gives a different type of finish.

 

[Alastairreid]

If you are practicing on a dry brick or block wall it will suck the life out of your render and when you come to float it up the sand comes to the surface will tear and fall off....dampen the wall down first and put plenty of plasticiser in the mix.

Sponging up is an optional finish as you say.

 

[cardigansand]

Building Sand is often much finer than rendering sand. I realise that the builders merchants didn't have specially designed render sand, but for the best and ultimate quality you really should source rendering sand. I don't know much, but I know sand!