Primers control the texture, absorbency and the colour of the support. It is important to use a good quality primer for good quality colours as the longevity of the painting depends upon all its constituent parts. Primers should not need diluting. First coats however, can be diluted by up to 10% to improve flow and penetration into the support. Primers should not be diluted to make them go further. Dilution will cause too great an absorbency and an underbound primer film, into which the paint will sink and be underbound itself. For a smoother finish apply two coats.