hello- I put mine in the saltwater and have a painted trailer. I use duct tape and tape up the midddle and back holes and then use salt way and file up the inside of the trailer. After that I pull the tape and spray inside all around each hole. Not the perfect solution but it works and should get most of it.

You could also dip it in a freshwater lake afterward if you have access. I put in at Redondo which has a spray area that I rinse my trailer right after the launch and spray inside each hole. Hope this helps.