Many professional artists advise against painting over a previous effort since most canvases, especially the widely available pre-stretched cotton duck, will allow an original image to shine through. Linen canvas is inherently more opaque, so fewer extra measures may be needed when overpainting on this type of canvas. If you are correcting mistakes as you go, you have the advantage of being able to scrape the paint from a portion of the canvas and revisit your efforts before the paint has dried. But if you are working over dried paint, there are a few steps you can take to enhance the final effort.
Things You'll Need
Gesso or oil painting ground
Fine-grain sandpaper or steel wool
Titanium white and carbon black acrylic or oil paint
Small paint roller or wide brush
Determine the workable area before preparing---does the entire canvas need to be covered over, or just a portion? If you are working with acrylic paint, lightly sand over the area several times with fine sandpaper or steel wool to remove texture and color. Oil paint can also be sanded over if the paint is completely dry. Brush or wipe away particles before proceeding to the next step.
If you are working with acrylics, mix carbon black and titanium white acrylic with gesso. The result should be a light gray, which will prove effective in creating a new opaque surface. For oil paintings, apply at least two coats of gesso designed for use with oil colors before painting over. You can also use an oil painting ground as a primer.
Apply the acrylic-gesso mixture to the canvas with a brush or roller; a roller will provide smoother results. Apply three coats of the mixture, allowing the paint to dry between coats. For oils on canvas, apply a solid color wash over the gesso coats or painting ground---some artists use brown or gray tones to create a more opaque surface.
Coat the acrylic canvas with a clear acrylic medium to seal in the layers underneath; once it is dry, you are ready to begin your new painting or touch-ups. The oil canvas should be ready for painting once the color wash, or base coat, is dry.
Experiment with "pentimento," or allowing the first images to show through subsequent ones. Some artists use the initial painting to create greater texture in the next, while others allow the images to show through added layers to create varied effects.
If you are covering up a textured canvas, try to sand out as much of the topography as possible before applying gesso or paint. Unless you want the background texture to be revealed in the new painting, sand the surface down so that it as smooth as possible before painting.