Fill knots and cracks in the pine with wood filler and allow it to dry completely. Sand the surface to produce as smooth a painting surface as possible. Wipe down with a damp cloth to remove all wood dust. Paint the surface with a pigmented shellac or oil-based primer product and allow this to dry thoroughly, before painting your top coat. The type of paint you use will depend on the type of sheen you want.
Paint washes can minimize the pine's grain, while still giving a hint that the surface under the paint is wood. This finish is common in Cape Cod style of decoration. This is known as pickling. You simply dilute paints with water to create a semitransluscent paint wash in a white or pale pastel shade. Experiment with the ratios of water to paint, until you get the coverage you are looking for. If the pine you are painting over is very light in color, you may want to apply a wood stain a few shades darker, prior to applying the wash. By doing this, the wash effect will dominate and not the wood grain.
Dye your pine with a much darker colored wood stain, such as espresso or black. This is another way of minimizing the grain. However, pine is difficult to stain, since it absorbs stain at different rates. Before applying the stain, treat the wood with two coats of water-based wood conditioner. Then paint on two coats of the dye, followed by two coats of shellac, before applying a glaze and top coat. This type of wood finish is labor intensive, but it is worth it if you have a good quality piece of furniture to work with.
If you are painting pine wainscoting, first you must determine whether it is flat or raised panel wainscoting. If your wainscoting is raised panel, paint the raised part the same color as your wall color and paint the flat panel in a coordinating accent color. This will give the panels dimension, yet hide the grain. Flat panel wainscoting can be painted in a coordinating color to the walls. If you want to create a dramatic effect, choose a color for your wainscoting that is the opposite side of the color spectrum to your wall color. If you want a subtle effect, choose white paint or a color that is a shade or two lighter than your wall color.