Out of curiosity, why are the supports rusting that high up though? His explanation made some sense that water is reaching the top of the support on the inside. Certainly no water has reached the outside that high off the floor.
Maybe the pole wasn't properly painted to begin with.
The only way the pole is filling with water, is a water episode from above. That would also be very hard as the pole is usually capped with a solid plate welded to the column.
If the pole was rusting from the inside out, you would be able to put a nail through the hole in the pole.
Just sand the rust on the pole. If you see bare metal underneath, it's rusting from outside the pole.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
B. Franklin 1759
That is only surface rust my friend. We paint them all the time in the shop with one coat of red oxide primer and when it gets to the site they drag the columns through the mud, dirt leave it out in the rain and snow and scrape it all up even before it get installed with mud still on it.