paint (something) with a broad brush
To describe or characterize something in very general, vague, or broad terms, ignoring or neglecting to include specific details. When asked about how she would stabilize the economy, the candidate painted her plan with a broad brush, resorting to vague claims about creating jobs and getting people back to work.
paint the lily
To add embellishment to something that is already beautiful or outstanding. The phrase comes from Shakespeare's King John: "To gild refined gold, to paint the lily ... is wasteful and ridiculous excess." My wife is so gorgeous that putting her in a fancy gown would just be painting the lily. Why add a filter to your photo of the rainbow? No need to paint the lily.
be like painting the Forth Bridge
To be such an involved or time-consuming improvement process that it never truly ends. The phrase refers to Edinburgh's Forth Bridge, which once required constant upkeep. Primarily heard in UK. Remodeling our house was like painting the Forth Bridge—once we saw how nice one room looked, we had to redo another!
be no oil painting
To be physically unattractive. Primarily heard in UK, Australia. Jack's a nice guy, he's just no oil painting, you know?