Dont do it !!
The concept of white render and never having to paint it is fine but in practice a white sand render is only ever white when its bone dry and new, when its wet its an array of unsightly shades and rarely has time to dry out white before the next downpour, it can at worse look horrendous.
if you want a white plaster finish then there are specific thin coat applications on the market designed for this purpose, a mixed application of sand and cement does not achieve the type of finish that you would imagine it does. Fine in a dash but not for a float finish.