I have tried many times to simply paint over the roughly painted texture. But I’ve found this to be nearly impossible. If you try to paint right over a highly textured area the corrected area will have a dry, leathery, chalky quality. So I have to scrape the dried paint until it is reasonably smooth. But you don’t need to scrape down to the canvas. A slight texture is not altogether undesirable. In fact, if you scrape too much you will get a surface that is too smooth.
To scrape the canvas I use a variety of ceramic tools. They are too dull right out of the package but I sharpen them with a common sharpening stone. I then strop them by applying a drop of chromium oxide green oil paint onto an old piece of leather. Make sure it is real chromium green oil paint and not a hue. The chromium oxide in the paint is identical to stropping compound and you can get a finely honed edge which will give you the control you need to very carefully scrape dried paint from the canvas. Be careful though, by using the chromium oxide green as a stropping compound the ceramic tool will be sharper than you might expect.
After the canvas is scaped smooth I brush a thick, transparent medium onto the canvas. Some artists call this “oiling up” or painting into a “couch” (a French word that is pronounced KOOSH).
I then paint into this layer of oil using paint that I have slightly diluted with a small amount linseed oil or walnut oil. This additional oil helps the paint to go on with smooth, facile strokes. Otherwise the paint can be too sticky and simply push the layer of oil around on the canvas. After you get a certain amount of paint built up into this layer of oil you can begin to add as much textured paint as you desire.
Although this technique seems ideal it has its problems and I have never been fully satisfied and try to keep its use to minimum. It has however saved many paintings that otherwise would be suitable only for the trash heap. When it is well executed it is almost impossible to detect its implementation.