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". Is this the same as plastering sand ? Would plastering sand be better in this case ?
Having left the new mix to dry out overnight I come back this morning to find large cracks have appeared throughout as it has dried out. Things are so bad I think I am going to have to start again. Cracks are worse at the edges where the wall meets the door frame (weakest point ?).
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 ? 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 ? 3) Is the 6:1:1 mix to weak ? 4) Have I overmixed ? I'm mixing up in a large bucket with a whisk attached to a drill. I guess you would normally use a cement mixer but as I am dealing with relatively small amounts I thought this would be OK. I'm adding the water first followed by cement, lime and then sand. Adding the lime certainly makes the mix more fluffy and easy to work with. Perhaps it's now too "bouncy" and full of air and as it dries out it's shrinking ? Mixing with a whisk is probably a tad more violent than a cement mixer. 5) I've been applying to about 10mm thickness. I have found the thicker I apply it the worse the cracks are but never had any problems with this thickness before adding the lime.
Any thoughts on this would be very much appreciated.