mr. Sharif,
as you probably know, Aluminium is very susceptible for porosity due to the behaviour of the self-protecting Aluminium-oxide layer.
This layer is often full of micro-cracks, wich can contain dirt/oil/water/etc. So cleaning is necessary.
Prior to welding, you should also grind the aluminium and weld it directly after grinding. The oxide layer is self-restoring when exposed to air or oxygen.
So watch out when you are pre-heating with a flame : you should regulate your burner to a neutral or reducing flame. An oxidising flame will restore the oxide-layer during pre-heating and will cause problems during welding.