Resistance welding of aluminum is complicated, due to a number factors, one of which is the difficulty in obtaining thermal balance in the weld because surface oxidation changes the dynamic contact resistance characteristics at the workpiece interface.  Aluminum has a low melting point and is very conductive, but the surface layer, aluminum oxide is just the opposite-- high melting temperature and low conductivity.

As the thickness of the oxide layer increases, the amount of welding current needed also increases.  Also, as the electrode makes contact with the surface, the oxide layer, which is very brittle, locally fractures, which creates only small areas through which the current can pass.  All of this makes for an inconsistent welding process.

So, surface preparation becomes extremely important.  Mechanical abrasion and chemical etching can both be used to remove the oxide layer on aluminum prior to welding.  Using them in tandem allows for maximum removal, in a short amount of time.  Using either one of them singly may result in incomplete removal of the surface oxide, but not necessarily.  Depending on your specific requirements (allowable processing time, cost, etc.) I would recommend using both sanding and etching prior to resistance welding.  Closely monitoring weld parameters like current, electrode force, electrode follow-up, etc. is necessary for consistent, optimum resistance welds in aluminum.  I hope this was helpful-- if not, please repost with more details: what conditions produced good welds, what conditions produced bad, etc.

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thank you  much