[nds1968]

In my experience with a spool gun you will have a cold start and you need to learn when to stop welding before it gets to hot. When you stop hold the spool gun there with the trigger half pulled so that only sheild gas comes out. This will allow the weld to cool while being protected by the shielding gas which will help prevent porosity. Also if your getting a crater at the end of the weld let it cool for a second and weld it again to fill in the crater (this is where your weld will want to crack).

Also to help prevent the crater put a tack weld down and weld towards and stop when to run into the tack weld.

The stainless wire brush is a must and as mentioned use it for welding aluminum only.

Preheating is also and option but I only do this if welding 3/16 or thicker (you would be surprised at the amount of condensation/water that is drawn out of the metal).

Pics may help.