I guess a big question is,.. are these bubbles.. rust, under chrome popping up,.. or hard debris, dryed to the tubes?...
A clue might be,.. are they on the front of the tubes,.. or all around, or mostly in the rear?.. If they're mostly in front area,.. it's likely things that have hit and stuck... you can buff that off...
If it's all over, or mostly in the rear, it;s probably rust coming up from underneath,... it's likely to worsen.. You probably will need tubes eventually...
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I was thinking dried bugs, and gravel pits... rust don't happen much here in Az, especially not on chrome...