Crack in rear passenger door
 

Someone left me this nice present for Christmas:

https://cimg8.ibsrv.net/gimg/www.rx8...671fba674f.jpg

That's not a scratch. It's an 8" long crack in the door (plus a couple paint scratches down below where the gifter's car door presumably hit mine).

I don't know anything about body work. Do I need to protect that against rust somehow? If I want it fixed, is my only option to replace the door?

That door is aluminium so no worries about rust.... but it should not have cracked from a door impact unless it was very cold outside or something.

I would suggest replacing the door, yes.

It hasn't been getting too cold here lately. I think it did drop down near freezing the other night and then got a lot warmer during the day. Maybe the temperature fluctuation did it in. Too bad :(

Are you sure you don't have aftermarket doors?

They look OEM to me, but I'm no expert on OEM vs aftermarket doors. If they're aftermarket that suggests one of the previous owners wrecked it and fixed it up without reporting it, since they don't look any different from OEM. That may well be the case. I'm planning to look at local salvage yards and on ebay to replace it.

Well no, there are fiberglass and carbon fiber doors for weight reduction. Both of which are more likely to crack than aluminum.

Even in low temps it shouldn't crack like that, consider getting in contact with Mazda NA.