The victory sets up a tie with First Division Newhaven on Wednesday 16th October (7.30) at Fort Road after they came through their First Round tie against Shoreham.
United looked confident throughout the contest and were close to taking the lead in the 26th minute, but Ryan McBride saw his effort deflected wide by the feet of Rustington ‘keeper McCarthy.
Six minutes later, United took the lead as a tempting cross by Scott Dartnall was only partially cleared by Chris Kirkham to the feet of Jason Taylor who slotted the ball low past McCarthy from 20 yards.
United had to wait until the 53rd minute for the second goal that their performance deserved.
McBride, who caused the home defence problems with his presence throughout the afternoon, was pulled back on the edge of the area and United were awarded a free-kick.
Jack Franklin stepped up and saw his goalbound effort deflected past McCarthy for his second goal of the season.
Linton Rogers should have effectively sealed victory three minutes later as a Taylor through ball left the Rustington defence static, but Rogers steered his effort just wide of the upright.
McBride soon made it three for United as he held off a Rustington defender to hit a left footed effort across McCarthy that nestled in the bottom corner.
McBride completed the United scoring after he was expertly picked out by an inch perfect floated pass from Howard Sturgeon and struck the ball home.
With the game into time added on, Rustington scored with their only shot on target of the afternoon.
Steve Kirkham found space behind the United defence and managed to find a way around Grant Young and saw his effort strike the post and go in, despite the best efforts of Andy Corrigan on the goaline.