The Englishman, joint leader with former Ryder Cup Swede Sandelin overnight on 13 under, remained level as the first three holes were parred by both players.
But with his birdie at the fourth the Challenge Tour graduate moved in front as he seeks to move into the top 115 on The Race to Dubai - he is currently 116th - to retain his card for next year.
Home favourite François Delamontagne was making a charge up the leaderboard as he raced into third on 12 under.
The Frenchman chipped in from a bunker at the fourth then holed a 15 footer at the next for back-to-back birdies.
And when he birdied the par five seventh he was only two off the pace, one ahead of Robert-Jan Derksen and José Manuel Lara.