2003 Hyundai Elantra GT: Take off Engine heat cover over Oxygen Sensor - Rusted

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2003 Hyundai Elantra GT: I want to change the Oxygen Sensor, upstream, top one, and there is a metal heat cover over it and attached to the manifold. **The bolts/screws are rusted/corroded in and haven't probably needed to be removed since year 2002 when the car was made. I was told by a longtime/experienced mechanic NOT to use an "Air, compressor gun" I tried BP Blaster and taking a LONG time!! **How can I get them out??