It's an old trick that is usually used with exterior paint. In fact, "Sandtex" paint has it already added.
Not a very pretty finish used internally I would have thought. I guess you're trying to improve pock-marked plaster. Maybe cracks and bits missing?
Try spreading fairly wet Polyfilla over the area with a plastering trowel/artex scraper/wide scraper. After it's dried a little, but still workable, flatten it with a large damp sponge. The smooth type of sponge that you get from cushions etc.