Will my cured undercoating be affected by RustBlast while treating the bare metal?

RustBlast will not disturb any fully cured coatings like your undercoating. You can do the KBS 3 Step System and do not have to worry about the RustBlast or KBS Klean disturbing your undercoating.

Check out the directions for the KBS 3 Step System here: http://www.kbs-coatings.com/rustseal-directions.html