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If your gonna have a problem with a skylight, it is probably already leaking.  I always ask the homeowner if the skylights have given them any trouble befor.  They usually respond that it never has or they had a bunch of leaks for years no one could fix and either the problem still exists or they replaced them. 

Try to keep the chem off of them.  You can rinse like like rest of the house but it is best to just keep it off.  Many chemicals used by powerwashers will etch the glass if not removed quickly.  If the skylight is domed it may be acrylic instead of glass.  Acrylic is very sensitive to ammonia so do not use glass cleaners on these.  Bleach should not harm the acrylic.

We always squeege them when we are done.

And to answer your question, yes we had 1 that leaked when we cleaned around it, so we sealed it up with a little through the roof.