Trust me, your problem is nothing to do with contamination caused by sandblasting. Take it from technical training and years of experience. You've got the idea, just keep your tung out of the puddle and filler off of your electrode

Try not to freeze your puddle so quick. I notice that in some of the photos theres a small nib so to speak. This is the result of dipping your filler, but extinguishing the arc before the filler can completely puddle in.

Hope it helps

One more thing