Plugging the holes in a bowling ball...

Just curious what others do to fill the holes in bowling
balls before decorating them for the garden or other
display.

I was just sitting here jamming aluminum foil into
the holes with a Sharpie pen and wondered what
others did.

Sometimes I use concrete, sometimes I use leftover
grout to plug the hole of another ball waiting to be worked
on. Just wondering.

:-) KM