Aluminium reacts strongly with the oxygen in the air to form a thin stable layer of aluminium oxide which covers the surface and stops any further reaction. You have to remove this oxide layer, by wire brushing, before you can braze the metal.
Aluminium is a soft metal, and the wire brushing will inevitably scratch and damage the metal surface. This is a good thing, because it increases the surface area for the brazing material to bond with the aluminum. You need to apply enough braze to fill up the "pores" that you create when cleaning the metal.
If the aluminium part was originally made by a "low tech" process such as sand casting, the surface will indeed be "porous" in the sense that the surface roughness from the original sand mould will still be there, and there may be some sand still embedded in the metal surface until you remove it by brushing. Of course the roughness tends to get filled up with dirt, as well as by the aluminium oxide layer, and becomes less obvious until you clean up the part.
The brazing rod melts at a lower temperature than the aluminium. The metal is hot enough when you can remove the heat source, and the brazing rod will melt when it touches the metal. Heating for longer doesn't achieve anything, and may melt the aluminium - which is not what you are trying to do!
If you haven't done this before, get some scrap aluminium and practice before you do the "real" job. The heating time depends very much on your heat source and the size of the aluminum part. It may only take (literally) a few seconds to get a thin aluminium plate hot enough to braze. Heating a solid block of metal with a small-sized gas torch will take much longer. The basic technique is to apply heat, remove the gas torch, apply the braze, and if it doesn't melt immediately apply the torch again and repeat.
If you don't have any scrap aluminium available, you can get some cheaply from places like waste disposal sites, scrap car breaker's yards, etc.
