I'm not familiar with P-22 aluminium. Is it a 6xxx alloy?
As said above, welcome to the world of porosities.
First you need to eliminate ALL sources of hydrogen and/or oxygen.
If, after you're sure you are working with clean materials, you still have porosities, you should try increasing the frequency with which you're welding. But first make sure there is no source of impurities from "outside" the weld (like on the filler rods, files, scotch-brite, wire brushes, the oxide layer, ...)