Originally Posted by
BK6128N
First. there is a needle-sized spot of rust on the seat tube, which looks quite deep and doesn't look like a surface rust to me.
Second, an area of rust is found inside the steel frame when peeking thru the holes of the main tube. I have no clue whether this only surface rust or harmful ones. (Zoom in and you'll find something shimmering inside the tube. Creepy)
Is there anything that I could apply to prevent it from rusting? (Boeshield T-9 isn’t sold in China, so buying it here would cost me an arm and a leg.)
That little spot of rust is just where the paint has been knocked off by something sharp, nothing to worry about.
Internally it does look quite rusty, although still superficial. If you could close the frame and fill it with rust remover (evap-o-rust, citric acid etc.) that would be good. Then dry it quickly/thoroughly and treat it with a corrosion inhibitor, it doesn't need to be Boeshield - we used to use LPS3 which is a spray-on sticky wax once applied, and it smells terrible so it must contain powerful chemicals.