I installed a Tange Seiki square taper bottom bracket couple months back and have noticed the spindle is developing some surface rust rather quickly. The FSA bb I replaced didn't seem to have this issue. I don't ride in the rain, but I think that the dirt on the road must hold moisture from morning condensation or previous rain and that gets flung up while riding and stuck to everything on the bottom bracket.

Other than washing my bike more often would it be beneficial to spray the spindle with wd-40 after every ride (avoiding the bearing area) to get the dirt off, or maybe applying oil to the spindle? My real concern is if left unchecked it could cause future issues removing my crank arms.

Thanks.

Easy fix is is just lay a thin film of grease over the axle. Common procedure for us old folks years ago. Also if you don't have a drain hole at the bottom of your BB shell drill one. Moisture will always collect in the shell and having a drainage hole keeps things from getting rusty. I don't know why any mfr. would sell a frame without a hole but that's the norm nowadays.