|| wrote:
I stained a cedar outdoor railing with an oil based stain. But there
are sections that keep developing patches of white spots. I can't
figure it out. I have sanded it to the wood I even stained on top of
it. I tried wiping it off with mineral spirit. When it is wet it look
like it is fine but when the mineral spirit dry or the layer of oil
based stain dries the darn white spots pop up again. I have no idea
what to do.

Sounds like something was previously spilled maybe that has been
absorbed into the wood--grease splatters from a grill, perhaps? Wood
being porous, if something else has been previously absorbed, it's
virtually impossible to get all traces that aren't on the surface off
so the stain is probably just not being taken up in those places.
That's only one guess, w/o seeing what the spots look like or any other
information it's pretty much a crapshoot as to what specifically is the
cause.

I can think of two things you could try--if take back to bare wood,
oxalic acid (wood bleach, most deck cleaners) might make a difference.
Otherwise, go to a non-penetrating solid stain to cover the surface w/
an opaque topcoat.