The result means United have hit six on the opening SCFL Division Two weekend for the second consecutive season, having defeated Storrington 6-0 last season.
It took 15 minutes for United to open the scoring as striker Ryan McBride struck before fellow new signings Jason Taylor and Adam Kneller made the lead 3-0 in quick succession.
McBride scored his second after the break in between a brace from Jack Divall but McBride was also dismissed late in the game.
Rustington scored a late consolation goal through Matt Wilson.
Next up for United is a trip to Lordswood in the Extra Preliminary Round of the FA Cup on Saturday 17th August. There will be a coach running from The Oval to the fixture, with limited seats available on a first come first served basis priced £