I would not think rain water would be a big issue.
It should be pretty free from metals....
But I would be more concerned with the other stuff that can accompany rain...lightning.

I would not think rain water would be a big issue.
It should be pretty free from metals....

The two things you'd really have to worry about are conduction and corrosion (which will lead to conduction).  If it's free of salts (i.e., metal ions) then a bit of water will be unlikely to do either -- I've got a receiver and ESC that stayed in a plane in a tree in the rain for a week before it blew down, the electronics are fine (the plane needed some work).

If you let it stay damp then the various metals will corrode, and the oxides will conduct electricity.  This will both be a problem in operation, and it'll be a problem because it'll lead to more corrosion.  If you're flying where there's salt spray then you'll also get salt on the works.  A day's flying probably won't cause you problems that day, but it'd lead to corrosion later.

So if it's just rain with no salt spray then go fly, and figure out a way to make sure that things dry out promptly once you're back at the shop.  I'd think on the lines of an ESC and timer that can be easily pulled out of the plane and allowed to dangle from their leads out the battery hatch.  If you live near the coast, then consider making ESC and timer removable so that you can inspect them for signs of corrosion and deal with it.

But I would be more concerned with the other stuff that can accompany rain...lightning.

In some places in the world you almost always get lightning with rain.  In others, lightning is rare.  I don't know about John, but I live in Oregon west of the Cascade mountains.  Here we rarely hear thunder and almost never see lightning flashes.  So flying in a light rain is probably quite safe (and some years, it's the only way to get in flying time!).  If it's the same where John lives, then he may be safe.

Back in the good old days I actually wrote the column Scoping Out for RC Car Action.  Mostly I reviewed speed controllers.  And yes if you got them wet they would blow the FETS.  The reason was that the FETS were switched hard on or hard off.  Both of these states dissipated very little power in the FET and they stayed cool.  Get them wet and the moisture mixed with the contaminates on the PCB would create a leakage path that would turn the FETS slightly ON.  In this state they would exhibit high resistance and at high current they would go POOF!!!

My engineering carear was in military/space electronics.  To qualify a military box it had to withstand 100% humidity and salt spray.  The primary way we accomplished this was by coating a PCB and all of it's components.

I haven't kept up with today's brushless motor ESC.  But I am certain they have FETS in them.  I also don't know if they coat the PCB.  Quit likely not.  So I would think water and ESCs don't mix.  They sell ESCs for boats.  I suspect that they are sealed.  It is also possible that some of the expensive speed controllers are coated - but I really don't know.  One of coatings we used was clear krylon.  If you are really concerned one could remove the shrink wrap - spray coat the board and components and replace the shrink wrap.  Just be careful not to get any of the Krylon in a connector.

Not much here you can hang you hat on - just relating the little I know about the subject.