For DNA Polymerase I (M0209), Large Klenow Fragment (M0210), T4 DNA Polymerase (M0203), and T7 DNA Polymerase (M0274):
To prevent formation of recessed ends (past the desired blunting point), pre-heat a water bath or thermocycler to 75°C. Before placing the sample at 75°C, add EDTA to your sample to a final concentration of 10 mM (to chelate the Mg2+ cofactor) then incubate at 75°C for 20 minutes.
Note that elevated temperatures, excessive amounts of enzyme, failure to supplement with dNTPs or long reaction times will result in recessed ends due to the 3´ → 5´ exonuclease activity of the enzyme.
Klenow Fragment (3'→5' exo-)
Inactivate by heating at 75°C for 20 minutes. EDTA is not necessary due to the lack of 3'-5' exonuclease activity.