FAQ: Can Thermolabile Exonuclease I be used to blunt dsDNA?

No. Short ssDNA ends are not a substrate. Use DNA Polymerase I, Large Klenow Fragment (NEB# M0210) to fill in 5′ overhangs and chew back 3′ overhangs, or use Mung Bean Nuclease (NEB# M0250) to chew back 3′ or 5′ overhangs. Note: 3′ overhangs cannot be filled in.
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    DNA Blunting Tutorial

    The first step in determining how your ends will be blunted is to determine if they are 5´ or 3´ overhangs. This tutorial will teach you how to identify what type of overhang you have, as well as which enzyme will blunt that end, and how.