Agriculture Pilot reported loss of control while attempting to use the Beta position to decelerate during landing. The aircraft left the pavement and damaged an airport sign.

Narrative

Performed wheel landing on RWY XY at ZZZ. On roll out went to use partial beta setting on torque lever to aid with deceleration. Had been flying an AT-402 with a PT-6 where I frequently used beta to aide with deceleration. Recently switched to an AT-402 with a Walter engine where it was relayed to me that the beta setting was difficult (stiff). More effort was required than anticipated to push the torque lever past the detent where the lever went unexpectedly to full beta which caused the aircraft to start a veer left. Tried to arrest the veer by applying right brake and simultaneously push torque lever out of beta. Was successful in pushing out of beta and prevent a ground loop but was; by that point; on a about a 30 degree departure from centerline. I then applied both brakes and stopped just off the runway surface. The aircraft spray boom which hangs approximately 1 foot from the surface impacted and damaged a taxiway sign. Damage was immediately reported to the FBO/Airfield POC (Point of Contact).