[header] How to render a wall [title] Chip away any old render and paint. [step] Use a cold chisel that is just sharp enough to scrape, chip away any bumps, dripping render, paint, or mortar that's stuck to the wall. Sometimes the old render may flake off and you'll want to scrape it all away before applying the new render. 
Continue to chip away at the wall until the stone or brickwork is free of bumps. [title] Scrub down the wall with a stiff bristled broom.