Will grooves on seatpost cause rusting inside frame? My bike has grooves in the seatpost as shown in the picture. The grooves do stop slightly further down the post, but the best saddle height for me is when the grooves partially go down into the seat tube. To me this looks as if water/rain can just flow down the grooves into the (steel) seat tube. What are the dangers of steel frames rusting from the inside out if water gets in? Is this seatpost designed to be used at a minimum height so that the grooves are completely above the clamp? (I can't see any minimum insertion height marked on the post.)
The best thing to do is apply a thin layer of grease to the inside of the seat tube and the outside of the post (only the part of the post inside the seat tube needs grease) and set the seat height according to what fits you.
The layer of grease will prevent the odd drip from causing rust.
If the bike is stored in a dry place after riding in the rain it will be able to dry out.
The bikes with seat posts rusted in that I've seen were the ones that were stored outside and never cared for.