Seatpost of a folding bike is always dirty, filed down by frame? I fold and unfold my Dahon Speed D7 at least four times a day, and each time its seatpost is being pulled up or pushed down the frame tube.

Problem is that the seatpost becomes dirty really quickly even on a dry weather. I suspect the aluminium it's made of is softer than the frame alloy and every time I move it up or down the tube it's being filed down a bit, and so the dirt is actually very fine chips of the seatpost itself. However, it hasn't been changing for a couple of years (shouldn't it already be polished enough by this moment?)

It's hard to tell for sure from the dirt look itself though. Is it possible for the road dust to get that quickly through the opening of the frame below? Would oiling the seatpost help or it would only make it worse (dry dirt is much more preferable to deal with than when mixed with oil)?

Maybe it makes sense to replace the seatpost with a steel one (not sure if they exist as a specific diameter and length are required).

Must be a common problem for folding bike owners, how do you cope with that?
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.