After having undercoated with Crown rustproofing done code P0451 turns on and off

It is unlikely that the undercoating caused this issue but we clearly can't rule anything out at this point.

Here is the testing that needs to happen in order to determine what the failure is. Let me know if you have questions but the normal cause is the sensor failed.

Hi,

It could be the Fuel Tank Pressure Sensor rust proofing should not have affected it. The is a test you can perform to conclude it but you will need a scan tool to read the the tank pressure voltage or a multi-meter and back prob the sensor connector. Check the FTP sensor voltage with the gas cap off, it has to read between 2.35 and 2.9 volts. If not, replace the sensor and recheck. I'm attaching wiring diagram and connector exploded view with repair guides below let us know if this information was helpful and what fixed the vehicle.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-emission-control-systems-work