2219 like 6061 and other heat treatable aluminum alloys are time at temperature sensitive. They are overaged by holding them at elevated temperatures. Their mechanical properties suffer as a result. The tensile strength, as welded, if done properly, is on the order of 35 ksi. The as welded tensile strength will not be as high as 35 ksi if the alloy is held at temperature for any length of time.

AWS D1.2 has a table (4.5) that tells you the maximum time at temperature for different heat treatable aluminum alloys. In your case, the time can be as short as 5 minutes at 450 degrees, 15 minutes at 425, 30 minutes at 400 degrees F, etc.

In the case of multiple passes or multiple repairs, the time at temperature adds up fast. The area affected is all the material bounded by the temperature ranges noted above. The "HAZ" can extend out a couple of inches from the actual weld.

You may have to consider cutting out the offending area and replacing the material with new and monitor your interpass temperature. Do not preheat the heat treatable aluminum alloys before welding.

If you are having problems with porosity, consider finding out why you have porosity and fix that problem first. It is usually one of two things that cause the problem; insufficient shielding gas coverage or insufficient cleaning or removal of any hydrogen containing materials such as oils, grease, finger prints, moisture, etc.

Best regards  - Al 

Is porosity in 2219 Al a reoccurring issue for your shop or was it limited to this one joint? If it is a reoccurring issue you may want to check out the NASA white paper on 2219 Al, it may offer some insight into possible practice modification, ( http://klabs.org/DEI/References/design_guidelines/design_series/1205msfc.pdf ). If the designed part is utilizing 2219 Al for the corrosion resistance characteristic of the base metal, heat treatment is recommended post weld where one is able to. From my experience if there is found to be an issue with an aircraft part that one is not able to replace than there are usually predetermined repairs for that part. In the case that the repairs specified by the T.O., (technical order), for the aforementioned aircraft part have been performed than the part will either be replaced or further assistance is sought. When the situation arises that further assistance must be sought either from lack of a repair or all repairs have been exhausted than the engineers assigned to that airframe are contacted. One will usually have a work around approved by QA, (quality assurance), when approaching the engineers or in the case no work around is able to be determined at a local level one will be supplied by the engineers. The internet is a great resource, but I would not complete any repair on an aircraft part strictly on advice derived from the internet. Any repair I preformed that deviated from the prescribed methods I would want in writing from an authorized source. I can say with certainty I would not want to stake that aircraft on the advice from any internet forum because it is your responsibility if that part fails while that aircraft is in the sky. There should be some sort of point of contact for the issues if it is a production or established repair shop.