The key is always: get rid of the loose parts.

The foam inside has to be dry and has to be consistent. It's function is holding the two ruddersides of laminate at the fixed and proper distance.
This will lead to the resistance against flexing of the rudder and resistance against impact on the sides by obstacles.

If the filler compound is thin, then the foam maybe can suck some filler compound. But the foam has to be non 100% closed-cell for that.
So in this case it will make the need for the dremel less.

But if the core is 100% closed cell (test it with a "suction-kiss"), you better dremel it out carefully.