Seatpost of a folding bike is always dirty, filed down by frame?
If you have an old inner tube available cut a section slightly longer than your seatpost. Slip it over the seat post and zip tie it in place. It will act as a somewhat water/dirt resistant boot, similar to what mountain bikes have on the front shocks. You may have to look for a fairly wide tube to fit over the post. Check with your local shop to see if they have any unrepairable ones left laying around.