If you scratch or gouge the inside of the mast with metal removal tools it may fail at those points.

I would try hammering sharpened plastic storm water pipes through the mast, repeatedly removing and punching out the foam core that will fill the pipe. After mechanically removing as much material as possible I would try high pressure water but I suspect the foam in the middle wont be removed by this method. In that case, with the mast vertical, sealed and with most of the foam already removed mechanically I would fill it with the cheapest solvent that has been proven to dissolve the foam, leave sit for 24 hours the drain it and blast it out with water.

This job will make a huge mess, especially the solvent aspect, I would consider leaving the foam if the performance impact is negligiable.

Interesting problem to solve using tooling available at home.

Darryn
Mozzy
1782