Here is an example of external render painting to a house that should not be effected by efflorescence any time soon. Without dealing with the issue before painting, chances are your render will end up having render burn.
What is efflorescence?
In short the render burn and efflorescence is cause by render that has not had time to cure and weather out. The lime in the render needs to escape and does so by being drawn through the paint. You will see white streaks etc all the time in painted render work.
Over the years all sorts of products have come out to help with render burn. At the moment specific lime lock sealers are what the paint companies recommend we use to avoid this problem.
You need to use a lime lock sealer prior to applying your top coats or leave the render at least 30 days, and wash down with water a few times prior to painting to help flush out some of the lime.
If your in doubt, ask your local paint shop for help, know the type of render before you go in store too. There are all types of bagged render and pre mixed.