ALuminum is a picky material....your porosity can come from a great deal of sources.     Grinding rocks ....really>?  I do not care who made them it is likely chunks of this abrasive material are getting stuck in your project.   Any kind of dirt or contamination in AL will always float directly to the top of your puddle and be trapped there.  A combustable containment could create a gas pocket and then visible pososity.  Overheating your weld pool both in time or heat can case hydrogen bubbles to appear at the surface of your weld.   Your best bet for moving aluminum material that is going to endure a welding process is machining.....by hand carbide burrs coated in boelube do a good job.  An Acetone wipe afterwards typically will remove any residue from the boelube.  Scotchbrite all surfaces afterwards. If you must remove material with a typical abrasive tool,,,,go back with some kind of carbide burr and do a thorough wipe down if you intend to get a reasonably clean weld on Aluminum.   For hand cleaning or even die grinder cleaning scotchbrite causes little harm to the welding process.  Aluminum is totally non tolerant of other materials in the weld pool.

877412     Before responded and got a discussion going I should have as usual just went along with Lawrence.  If you do a search on Aluminum prep or Cleaning or porosity you will get several very in depth threads that cover the subject well...beat it to death even.  Good reading and ideas.

The material used to manufacture the grinding disk or stone may not be a source of hydrogen, but it is impossible to count the times I've seen people use bee's wax on the tools to prevent them from getting "loaded" with aluminum particles. What comes next? Welding of course and the resulting porosity!

Al

Frank Armao also says:

"Minimizing porosity is mostly about cleanliness and housekeeping. There are actually many possible causes of porosity. Following are the top four culprits and how to prevent them from happening.

    Poor Preweld Cleaning. Be sure to degrease the part before you weld. Wipe down the surface of the material with a clean, lint-free rag saturated with a good degreasing solvent. Acetone and toluene are among the best. Alcohols are not good degreasers and should not be used for preweld cleaning. Following degreasing, the aluminum surface is then deoxidized, usually by using a stainless steel wire brush. Mistakes here are the most common cause of porosity.
    Insufficient Shielding Gas Flow Rate. For gas metal arc welding (GMAW), the flow rate should be at least 35 SCFH, although flow rates up to 60 SCFH can be used, especially with big gas nozzles. Argon/helium gas mixtures require even higher flow rates. For gas tungsten arc welding (GTAW), the flow rate should be at least 25 SCFH.
    Incorrect Nozzle-to-Work Distance. Simply put, this distance should be as small as possible, preferably in the range of ½ to 5/8 in. Longer nozzle-to-work distances can suck in surrounding air and other contaminants."
    Improper Gun Angle. Aluminum welding always requires a leading gun angle. If you try to use a drag angle or even a 90-degree angle, you will get porosity in your welds.

Some people can't handle the truth. LOL
   M.G.