Portland’s Jeremy Rivera turns a double play against the Trenton Thunder on Thursday in Portland. Sliding into second base for Trenton is Bruce Caldwell. (Shawn Patrick Ouellette/Portland Press Herald Photo)

PORTLAND — Gargle. Rinse.

Don’t repeat.

Daniel McGrath washed away the sour taste of his last outing with a sparkling six-inning effort Thursday afternoon to lead the Portland Sea Dogs to a 3-2 victory over Trenton to complete a three-game sweep of the series between the Double-A affiliates of the Red Sox and Yankees.

A matinee crowd announced as 5,496 at Hadlock Field saw the cellar-dwelling Sea Dogs extend their winning streak to four, matching a season high.

Six days earlier in Akron, McGrath walked four batters in a row and all scored. The left-hander from Australia did not make it out of the third inning.

“He never gets hit really hard,” said Sea Dogs pitching coach Paul Abbott. “His issue has always been control and command.”

Abbott noticed McGrath’s arm slot dipped when throwing his change-up, which tended to flatten out the pitch as well as tip off opposing batters that something other than a fastball was coming. They worked on correcting that flaw as well as sharpening a cut fastball introduced at the end of July, expanding his repertoire to four pitches. (He also throws a slider.)

“He picks things up pretty quick,” Abbott said.

Thursday marked the longest of McGrath’s 30 appearances this season and was his eighth start. He held the Thunder – currently in second place in the Eastern division of the Eastern League – to three hits, striking out four and walking two.

“It was a lot better than last outing,” said McGrath (2-3, 3.59). “They were swinging early. They were trying to be pretty aggressive on it, so I was trying to help, throwing strikes down in the zone and they put the ball in play.”

A streak of 15 consecutive scoreless innings from Portland starting pitchers ended in the second when Trenton’s Jorge Saez doubled to drive in Gosuke Katoh, tying the game at 1.

McGrath retired the next nine. Thanks to a pair of double plays, he faced the minimum three batters in four of his six innings. Eleven of his 18 outs came through the air. Factoring in four whiffs leaves only three groundouts.

“My change-up and my cutter are my two best pitches,” he said. “It kind of gets people out in front of the strike zone so they get underneath the ball a little bit more. That’s why I get a lot of pop-ups and fly balls. I’ve always been like that.”

Chad De La Guerra snapped a 1-1 tie when he led off the sixth with a blast into the fourth row of the blue pavilion seats in right field to give the Sea Dogs the lead for good. With the count 3-0, De La Guerra looked to Manager Darren Fenster, and Fenster signaled to swing.

Trenton starter Nick Green, making his Double-A debut, had worked out of trouble with help from three double plays and allowed one run through five innings before facing De La Guerra for the third time.

“He hadn’t been leaving too much right over the plate,” De La Guerra said. “So at 3-and-0, I was looking for him to mail one right in there, and he did.”

The home run was De La Guerra’s 12th of the season. Michael Chavis reached on a throwing error and eventually scored on a bases-loaded walk by reliever James Reeves to make it 3-1.

Trenton got a run back when Jhalan Jackson homered over the Maine Monster to open the eighth against reliever Harrison Cooney with a blast estimated at 420 feet. The home run was Jackson’s 15th of the season.

That was the only hit allowed by Cooney in two innings. Adam Lau came on for a perfect six-pitch ninth to convert his second save.

The Sea Dogs had jumped ahead 1-0 in the first when Jeremy Rivera beat out a grounder to the shortstop hole, advanced on a sharp single to center by Chavis and scored on a two-out single to right-center by Luke Tendler.

The homestand continues Friday night with the opener of a three-game series with Altoona.

The Portland Sea Dogs celebrate their victory over the Trenton Thunder on Thursday in Portland. (Shawn Patrick Ouellette/Portland Press Herald Photo)