Is “sand off the sharp edge” a well-established idiom?

"Sand off the sharp edge" seems like a variation on "smooth off the rough edges".

"Smooth off the rough edges" is a reasonably common metaphor, and does not always use the exact phrase.

The metaphor is that of a woodworker, smoothing a rough piece of wood with sandpaper, resulting in a piece that's acceptable to more people.

"Sand off the rough edges", "Smooth off the rough surfaces", "Smooth off the edges", and indeed "sand off the sharp edge" are all variations on this.

It can be complimentary:

"I was not impressed with John's essay at first, but now he has smoothed off the rough edges and I like it more."

But sometimes it can be negative:

"Diana was an interesting act when she started, but her management have smoothed off the rough edges so much that she's boring".

It seems to me that the example used in the question, deliberately emphasises the ambiguity of whether the "sharp edges" used by Obama are a good thing or a bad thing. A sharp knife is both useful and dangerous.

I think it is being used in this form as a political comment. The implication is that he has been honing - sharpening - some of his tactics, rather like knives, ready for a fight.

If he is sanding these off, he is making them blunter or smoother, meaning that his words and actions are going to be smoother with others.

No other buttons seem to work so this seems the only vehicle right now to make a comment. What is a bit disconcerting to me is the question title's very reference to idiom and, inferentially, to American lexicon--which led me to believe the source was misunderstood. I see nothing about the saying that is idomatic: all words are well defined and commonly understood. Their presentation as a simile, alone, is not (yet) commonplace--in fact seems largely an impromtu literary creation.

I would agree that the expression, as colorful as its imagery is, is neither commonplace nor a significant, if at all, subset of the American idiom.

I would offer that it is best understood by understanding the circumstances and surroundings in which it is couched.

The State of the Union speech delivered yearly by the President occurs inside the District of Columbia (Washington, D.C), inside the Capitol, inside the House of Representatives, before a joint session of Congress.

Outside of the Beltway refers figuratively to the United States at large as opposed to within the capital city--which is posited figuratively to be situated inside an Interstate highway loop, along U.S. Interstate 95 (mostly in Maryland), which more or less rings the capital city--more specifally, "inside the beltway" is a kind of insider catch phrase referring to the headquarter institutions of national government.

The notion of sanding rough edges (essaying to be less sharp...as in less critical) during a speech before his own and adversary party legislators is merely to suggest--and "suggest," indeed, is the tone taken in that article--that the orating President, while claiming to be working towards interparty rapproachment and cooperation, will not want to be seen as overcritical with regard to his adversary party, as had been his wont during other speeches across the country (outside the Beltway).

Said another way, the column writer might have said, simply, that the President, before Congress and a near worldwide TV audience, will see it as politically expedient to assume a less combative, more conciliatory posture as regards bipartisan interaction on legislative matters.