some let them soak ( i never have) i use Ajax or Comet and lightly scrub parts then wash in warm water. some casters don't even use a releasing agent . u can always tell if its not clean as the water will bead right off..
its what we call a Go Devil down here in the swamps of Louisiana..it will propel a boat through a mixture of mud and water and is primary made for Duck hunters as the tide rise an falls..
this all started with the frame of a ford 8000 , i added a pulling front axle with old school military grip tires, scratch built bumper and a 1160 Cat under the hood..boat, motor and trailer is scratch built
i use real dirt mixed with elmers glue myself , but if you are happy with the results then it works..maybe i will try that with my weatherning pastells..