Graham Lockhart from Australia sent me this tip showing how he's sanding the curved side of the sponsons and keeping it 90deg to the top. Thank you for the great tip Graham!

"I found a way of sanding the outer foam on the sponsons a bit easier, it also keeps the sanding at 90 degrees and does a great job, i also have used a dark pencil to mark the timber so not to sand the wood when shaping the foam"