Harrow produced a dreadful display to slump to another defeat ahead of Christmas.
While the Earlsmead pitch was heavy, it was playable, as Josh Andrew returned from suspension and Excellence Muhemba partnered Marc Charles-Smith in attack, in two changes from the previous weekend's loss at Gosport.
The game wasn't four minutes old when Harrow lost the ball in the far right corner of the ground and Weston's Vincent Harper picked it up, shook off various half-hearted challenges and stormed 60 yards or so down the left wing, before playing the ball inside to Bradley Ash, who went through the centre of Borough's defence to fire home.
Harrow heads dropped immediately and James Waite lifted another effort narrowly over Luca Ashby-Hammond's bar, before Shaun Preddie made a fine challenge to prevent another shot, and Ashby-Hammond kept out another effort from Ash.
Harrow's first attack of any seriousness came after 20 minutes, when it took a good tackle on Muhemba to stop him making clean contact on a George Moore pass.
At the other end Scott Laird curled an effort over the bar before Borough conceded again after 33 minutes. Michael Bryan slipped as he turned just outside the Weston box and, as with the first goal, as soon as the visitors won possession they were able to sweep forward virtually unchallenged, with Waite being left at the end of the move with an easy finish.
Half-time arrived with virtually no response from the hosts to that second goal, but early in the second period there was some life from Harrow, with some better possession in their opponents' half.
However, it was Weston who went closer to scoring as Dan Martin's shot whistled just over the angle.
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Harrow made a double substitution and, while it was a good idea to try to introduce some new life, not for the first time lately the substitutions resulted in an immediate goal for the opposition, as a cross came in from the right, was only half-headed clear, and Waite lashed home the dropping ball.
Luke Purnell in the Seagulls goal had to make his first save of the afternoon as he dived bravely at Moore's feet.
Moore then shot narrowly wide from Dylan Ive's pass, and saw another shot beaten away by Purnell, but Weston, who began the game only three points ahead of Harrow, were able to manage their lead very comfortably.
And in the first minute of added time Waite ran through on the left, rounded Ashby-Hammond and slotted in his hat-trick goal.
Harrow had drawn at Weston less than eight weeks earlier, but now sit fifth from bottom after taking just nine points from the last 51 on offer.
The last five league games have seen two goals scored (one by Dylan Kearney, who is no longer at the club) and 13 conceded and the two teams below them are each only two points behind with three games in hand.
Boxing Day sees them travel to one of those two sides just below them - Hendon - for a north London derby.
Harrow: Ashby-Hammond, Andrew, Haugh, Pepera, Preddie, Ireland, Moore, Cole (Donnellan 69), Charles-Smith, Bryan (Ive 64), Muhemba (Akintunde 62). Unused subs: Otudeko, Fenton.