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You won't find a harder place to paint than behind a toilet.  Sometimes there are only a few inches of space to work in, which isn't room enough for a brush or standard roller.  

There are special tools to paint in tight spaces.  Wooster makes a product called the Mini Koter.  It is a long-handled roller frame with a low profile roller.  This extended handle allows you to paint behind objects without having to get into the tight space yourself.  They can also be used to paint walls like a regular roller cover.  Using a Mini Koter means you won't have to use an extension pole, as the handle will allow you to reach right up to the ceiling.  

Check out the Mini Koter here