[Goemon]

A flu cutter is not an appropriate tool for surfacing a spoilboard and is not safe in any router spindle, unless you can use it under 1000 rpm.

You want something like this.
https://www.amanatool.com/rc-2255-cn...outer-bit.html

Or even a plane carbide tipped router bit with a large diameter.

Thanks. I've ordered a pair.

Out of interest, is it the spindle speed that makes a fly cutter unsafe (I.e. that the carbide cutter might come loose and get launched at me) or is it the material (I.e. fire risk with wood)?

My spindle actually can go as low as 1000 rpm (or at least the manual said it's rated for that). It's electric fan cooled but it doesn't pack much of a punch at that speed.