Arjay transom pour is designed to be a pour in place transom product that sticks to just about everything.
Using a chainsaw to remove the rotten wood is an excellent technique if performed correctly. The goal is to remove as much damp and rotten wood as you possibly can. Once you have removed an adequate amount of the existing core material, make sure the transom is completely dry before applying Arjay.
Using a space heater or heat gun help get the transom bone dry. If the inner core area is too difficult to access, you may want to let the boat sit for a few days to allow the core to air dry.
The drier the transom, the better the Arjay product will adhere. Small traces of water and oil are ok. Just ensure that the inside of the transom is mostly dry before you begin pouring.
Also, make sure to seal any existing holes in the transom to prevent the product from leaking out during the pouring process.