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.)
1An advantage is if it does corrode, its easy to get chemical solutions where its needed.– mattnzDec 1, 2020 at 18:58
The reason for the grooves/internal ridges in your seatpost is likely to stiffen it. Just as with corrugated cardboard, having ridges stiffens the material against longitudinal bending.– ArmandDec 1, 2020 at 22:49
2I’m not a vintage parts expert, but this may be called a fluted seatpost.– Weiwen NgDec 2, 2020 at 1:16
@Armand So a cardboard box is stiffer than a sample of solid wood of identical exterior dimensions?– Phil FrostDec 2, 2020 at 23:12
@Phil Sometimes, and usually lighter. The point is that adding such ridges increases the stiffness, so you can get by with less material. For your specific example, I think a balsa wood box would be less stiff than a corrugated cardboard box (when trying to bend perpendicular to the corrugations).– ArmandDec 3, 2020 at 9:36
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.
2And it's best to take out the crank axle assembly and grease the entire inside, plus the outside of the crank cartridge, if there is one. Dec 1, 2020 at 15:42
Another option is to cut a longer length of old inner tube, and put it over the seat post before mounting to bike. Then once the height is right, lower the "skirt" over the top of the clamp.
Its not perfect on your bike because there's little "throat" above the clamp, so for you there will be minimal overlap. Sorry.
Aside - your bike has a pinch slot for the seatpost clamp, which is another open area, perfectly aligned to catch road water off the back wheel. So don't forget that ingress point.
Lastly, make sure your Bottom Bracket has a clear drain hole - water does more damage when it sits, so if you can let it fall out the bottom that will reduce problems with standing water inside the frame. Storing the bike inside helps, as does keeping it away from cold.
Much sooner than any corosion you will have problems from sand or other mud particles getting between the seatpost and the seattube and causing horrible clicking with every pedal stroke or when shifting your weight on the saddle.
You can try applying the trick I employ, having myself big proplems with that even without any grooves. So far I used electrical tape to seal what I can but this sealing instruction appears to be more robust https://www.bikeradar.com/features/budget-bike-tech-seal-your-frame-from-mud-and-water/ (uses silicone and a rubber o-ring).
You could use silicone caulk to seal up the grooves. You can probably find caulk in black which would blend in. You want caulk that’s designed for kitchens, bathrooms or exterior that seals and resists water.
4Most silicone caulks/RTV release acetic acid (vinegar) as they cure - probably not the best for preventing corrosion.– ArmandDec 1, 2020 at 22:46
In addition to that, the silicone caulk will come loose if you ever move or remove the seat post, for example to accomodate a different rider or riding style - so possibly not a sustainable solution.– sleskeDec 2, 2020 at 8:25