My Apollo folding bike has an annoying issue with the seatpost/seat tube interface. The seatpost fits nicely into the top of the seat tube, but when it gets below a certain point, it becomes extremely stiff and has to be forced down. This wouldn't normally be an issue, but the folding mechanism for the bike involves dropping the seatpost, and it's taking a lot longer to fold the bike than it otherwise would. I've tried flipping the bike over and inserting the seatpost from the bottom of the seat tube, and it starts out stiff when I do that, so I think it's the frame not the seatpost that's the issue.

The stiffness seems to begin when the bottom of the seatpost is level with the area where the main tube of the frame is welded to the seat tube, so I suspect it's got something to do with distortion during welding. I know it's a cheap bike, so this isn't a massive shock, but what can I do to fix this? The frame is aluminium, is there a way to ream out the seat tube?

Thanks in advance, and sorry for the huge post. I seem to say that a lot on here....