I have written down some reasons of porosity in aluminum welding. See below.
Aluminum has a oxide layer over it called aluminum oxide which protects it from corrosion. This layer is very thin, you cant see with naked eye. This is porous in nature. I t can absorb contaminants in it which creates porosity. What can be absorbed in this layer is oils, paints, grease, dust and moisture. Remember what causes porosity is typically hydrogen, liquid aluminum likes hydrogen very much. If any hydrogen is present around weld metal it will be dissolved in large amount i guess the value is 0.69 cm ^3 per 100 gm of molten aluminum. When that liquid aluminum starts solidifying, hydrogen does not wants to stay in anymore, it leaves the solidifying weld puddle until solidification is complete, now in solid form solubility of hydrogen is 0.036, dropped by 20 percent. This big difference in solubility creates porosity.
You have to cut down the sources of hydrogen to prevent porosity. First you have to know what could could be the sources of hydrogen then know how to get rid of them. I know some of the sources are
1) Paints, oil and grease are hydrocarbons, they contain long chains of hydrogen and oxygen. If they are present on the joint surface or filler rod then they will break under the heat of arc and hydrogen and carbon will be separated resulting in dissolving of hydrogen in liquid aluminum.
2) Moisture is a source of hydrogen because its formula is H2O. Under the heat of arc it too gives hydrogen, Sources of moisture are
a) Moisture in the shielding gas.
b) Leakage in shielding gas hose can draw moisture in.
c)For a given value of Relative humidity and room air temperature there exists a temperature that if anything in the room attained that temperature then moisture in the atmosphere will be condensed on that. This is called dew point. For example, in a room in which welding is to be done, RH is 80% and air temperature is 70 degree Fahrenheit,, corresponding to this the dew point is 63 degree Fahrenheit . Now, if your base metal,filler rod has a temperature less than this dew point value so surely moisture in the air will be condensed on the surface hence generating a source of hydrogen. Be aware of dew point.With increase in RH and air temperature the dew point increases.
Welding procedure for preventing porosity:
Now you know the reason is hydrogen typically, you need to cut down its sources.
1)Purchase high quality shielding gas 99.997% from an approved supplier with dew point of -76 degree Fahrenheit for argon and -71 for helium .
2) For welding you first need to remove the oxide layer. You can remove that layer by immersing filler rod and base metal in heated solution( 70 degree centigrade) of 10% NAOH in water for one to two minutes followed by 50% HNO3 and 50% Water solution. This will remove oxide layer and contaminates too. But remember oxide layer forms immediately but its very thin. It grows to full thickness in 24 hrs so you need to weld quickly.
3)After cleaning, just prior to welding, stainless steel wire brush your joint surfaces, this will remove oxide layer too. Point to be noted is never wire brush joint surface prior to etch cleaning. This will smear the oxide layer containing contaminants which will cause defects like porosity and LOF.
4) Use 90% iso porpyle alcohol to clean the joint surface and filler rod.
5) Never touch joint surface with hands as your skin is a great source of hydrogen because it contain oils.
6) Cover joint surfaces with brown Kraft paper or polythene bag or cheese cloth.
5) Set optimum shielding gas flow rate, not too high, not too low. Both will cause porosity. Low will be insufficient and allow air inside. High will draw air in.Qualify the procedure.
7) Maintain leading GTAW angle of 15 degree from from vertical and feed filler rod at 70 degrees from horizontal. Too much decrease in angle can cause porosity.
8) Higher arc length can cause porosity.
9) Clean all the tools prior to use on aluminum. Tools like files, grinders ( not aluminum oxide) , ss wire brush. They must be cleaned with iso propyl alcohol.Tools should be dedicated for aluminum only.
10) Do not weld over bandsawd surface as it may have smeared oxide layer.
11) Store filler rods vertically to escape possible dew point condensation.
12) Pay equal attention to filler rod cleaning.
13) Use clean pair of gloves because your filler rod touch with that. GTAW gloves are made specially by using processes which do not use oils in their manufacturing.
14) Train welder so that he must not take heated filler rod out of the shielding gas zone.
15) Avoid excessive spatter inside gas nozzle.
16) Avoid welding in wind drafts as they may effect shielding gas.
17) Try to weld fast.
18) Handle all joint surfaces and filler rods with clean nylon gloves every time.
19) Use files and ss wire brush in forward stroke.
20) If using power rotary brush instead of manual ss wire brush then do it with light pressure.
21) Grinder recommended for aluminum is silicon carbide with 30-50 grit size and 4000-5000 rpm.
22)Always clean the joint surface also with alcohol mentioned above before using any tool like files, ss wire brush and grinder. Any contaminant present on the joint surface may contaminate the tools and then furthur working will contaminate both.
I know only these. May be others have more to say or correct me if i am wrong.
If you have nothing to go off of but to finish what the man before you started, I'd stick with qualifying IAW ASME Sec IX. Although, maybe that's why your man before you is no longer there. 
About the porosity issue. As said above, you should find some good info on this subject because at one point in time I was involved in a thread that had a wealth of information on this subject. To be quick and to the point on what you should be looking for and what I found to have been a source to my porosity using the same base material and filler, moisture/condensate coming out of the material during welding, machine setting as in to much A/C frequency adjustment and the cleaning process such as using acids to clean that leave residuals on the material that get vaporized during welding. I'm sure you are aware about how clean the ally needs to be before welding.
