I want to repair a couple of cracks in the rendering on the rear wall of my house. I've removed the rendering around the cracks to a width of about 30mm and a depth of about 10mm. The wall is sound behind the cracks which appear to have been there a long time. The removed material appears to be just sand & cement, which I guess was applied when the house was built 40 years ago. The surface of the rendering is rough, but there are no stones in it, is there a technique I should use to get the rough look finish so I can blend in the new rendering with the old? I'm painting the wall with masonary paint when I've finished so the colour doesn't matter. What mix should I use, and sharp or builders sand or a mixture?