My suggestion for fully rendering something is to fill the layer first, rather then painting on an empty layer. Just pick a background color, even just the canvas color, if you wish, and then hit ctrl+shift+F or the little FILL button. This is, if you wish to work similar to acrylics or oil, though, it works either way. There are interesting things happening, if you don't do it, but favor more watery styles, but you really have to warm up to what's happening then (cough, cough).