Why do you need to stir paint before using it?

Stirring for 3 -4 mins. is preferred (especially heavy paints like primer) since shaking can introduce air bubbles which will have to be stirred away before you apply the paint to the surface you are painting, otherwise pots marks will appear and a second coat will be necessary to conceal the blemishes. Frequently, when asking someone at the paint counter to stir a couple of paint cans for you they will place all of them on a shaker machine to save themselves some time and trouble - insist on the centrifugal machines instead. Happy painting!

BTW - Use a halogen work light to identify problem areas before painting and during application of paint. I prefer to paint out of direct sunlight and use 1 or 2 halogen work lights for illumination and great results.