One point that is often overlooked on a bicycle is the top of the seat tube were the seat clamp or binder bolt holds the seat post in position, here is also a split in the top of the seat tube that allows you to compress the seat clamp. Over winter this point fills up with water, grit and salt sprayed up off the back wheel while in motion.
So how to stop grit getting down your crack ?
Protect yourself with a seat post condom - no don't rush off to your bedroom and grab a multi ribbed colorful condom although once cut to size it would certainly work I'm sure. Most likely you will have an old bicycle tube laying around, if not ask the local bicycle shop for an old 700 x 28c tube and cut a 50mm section off it and then turn it inside out, give it a clean. Remove your seat post & now is also a good time to wipe it down and apply fresh grease to the seat tube. Now with your freshly cut 50mm section of bicycle tube stretch it over and down on the top of the seat tube below the clamp. Now re insert your freshly greased seat post, set it to the correct height. And now you can pull up the post condom and cover the area that needs protection. Once the warm rays of spring arrive you can just cut it off or leave it on there for all year long protection.