For the most part, preparation for exterior surfaces is identical to preparation for interior surfaces with the following notes.
New and tannin rich exterior timber should be allowed to weather before painting. It's then a good idea to sand down or wire brush to remove dead fibres and open timber pores.
With the roof, colourbond and galvanized iron needs to be cleaned and sanded, while cement tile roofs should be high pressure cleaned and made free of any loose particles.
For new concrete, cement render and mortar, the surface should be allowed to cure for 28 days before painting. For previously painted concrete surfaces loose and flaking paint needs to be removed, glossy or enamel surfaces thoroughly sanded, then the surface cleaned as with all the other surfaces. Repaint work can be a little tricky and will likely require a primer.
See the section on undercoats for more information. Often old oil enamel paints become very brittle and crack after a few years outside. Any peeling and flaking paint needs to be removed by sanding, scraping and/or stripping. Next it needs to be cleaned and dust removed from the substrate using detergent such as sugar soap.