Hi Nev, it's Ja here, not Joe (Joe's username is CAPTSLOW).
This is an idea you can try (assuming the foam is conforming with the inside of the extension - ie: tight gap between inside diameter of extension and foam). I use this trick with aluminium booms (which I don't own/use anymore). Dismantle boom and remove all sand. Dry boom and spray inside of boom and extension with silicone spray. I used this stuff to stop of ali booms welding up, but I noticed when doing this, water didn't enter in the boom (and when I think about it, sand for that matter as well). Anyway, just an idea since nobody else has offered. The only thing I hate about doing this is sometimes some of that silicone ends up on the grip and make the boom hard to hold. You don't need to apply a lot of silicone though.
Has Frank been out ?

... hang on, don't answer !. He can't have because everybody has been sailing