The South African off spinner bowled 16 overs unchanged to pick up two more wickets to add to the 20 he has already taken in Essex’s previous matches against Kent and Surrey.
He sent down nine maidens and removed skipper Ben Brown and Delray Rawlins with Sussex 155 for six from 61 overs when rain drove the players off just before lunch.
The rain eventually gave way to bright sunshine but after a second inspection at 5pm umpires Ian Gould and Mark Newell decided there would be no further play because of a wet outfield with the run-ups a particular concern. Just 31 overs had been possible.
With two days left it is difficult to see how south group leaders Essex can chisel out a third victory out of three, although while they have Harmer there is always a chance.
The 31-year-old’s 16 overs cost just 21 runs and although only the occasional delivery turned off the straight, Sussex’s batsmen – most of whom were encountering Harmer for the first time in red-ball cricket – could do little more than survive because of his consistent line and length.
Sussex skipper Brown looked to attack and came down the pitch to deposit him beyond mid-on in the 41st over after Harmer for once dropped slightly short, but Brown departed in his next over when attempting to hit over the top again only to offer an easy catch to Simon Walter at mid-on.
Rawlins was similarly determined not to get tied down but he mis-judged a sweep, the ball looping up off his glove to Alastair Cook at slip for seven.
There wasn’t much respite at the other end for Sussex who had resumed on 93 for two under overcast skies and with the floodlights quickly in use. A slow outfield didn’t always give batsmen full value for their shots either.
Jamie Porter, who removed both Sussex openers on a rain-affected first day, was as effective as Harmer in drying up the runs.
He picked up his third wicket of the innings in the third over of the day when Harry Finch (21) edged to slip attempting a forcing shot off the back foot. Porter sent down six overs in his pre-lunch spell at a cost of just seven runs with three maidens.
Teenager Tom Clark took 30 balls before he got off mark with a thick edge off Porter to the third man boundary but the 19-year-old, who made his maiden first-class half-century against Kent last week, looked the most secure of the Sussex batsmen, defending solidly against Harmer and looking for scoring opportunities against the seam bowlers.
But having survived for 97 minutes and reached 21 from 82 balls, the 19-year-old left-hander was superbly caught low down at backward point by substitute fielder Feroze Khushi for 21 to give Aaron Beard his first wicket.
Aaron Thomason, the former Warwickshire all-rounder playing only his fourth first-class match, has so far resisted for nearly an hour for 14 while George Garton is unbeaten on eight, but it was somewhat ironic that while his team-mates struggled, Laurie Evans made 65 during his second game on loan for Surrey against Hampshire a few miles away at Arundel.
Clark said: “It was a great experience to face some really good bowling on a pitch that was doing a bit. When I came in on the first day it was very tough and it was the same again today against the likes of Simon Harmer and Jamie Porter who have so much experience.
“The only way you are going to learn is to face the best bowlers so I was pleased that I managed to stay out there for quite a long while and get some runs. It is great to be getting this opportunity [at 19] in this competition. Jason Gillespie [Sussex Head Coach] is trusting us during these games to play with freedom and it’s been very enjoyable.”