Friday, 14 September 2018:

SCOTT MCLAUGHLIN driver of the #17 Shell V-Power Racing Team Ford Falcon FGX looks on during qalifying for the Supercars Tasmania SuperSprint in Hobart, Australia.

Reigning champion Jamie Whincup and veteran co-driver Paul Dumbrell have stamped their authority over the Supercars field ahead of the Sandown 500.

The pair dominated practice in Melbourne on Friday, with Dumbrell setting a new lap record while Whincup also mastered the layout before conditions deteriorate.

A tricky qualifying test awaits all drivers on Saturday, with teeming rain and perhaps even hail forecast.

But on Friday's evidence, it's car No.1 that has the speed and driver pedigree to win and win well in the first of a block of three endurance tests.

"Minimising mistakes, that's the key," Whincup said, after racing to the front of the field.

"That's where we've let ourselves down in the last three or four years. We've always had a quick car to do the job here but we've made errors.

"We're not going to get carried away with car pace ... we'll keep our concentration and keep our head down."

Last year, a puncture skittled their hopes of a podium finish and in 2016 they were undone by Dumbrell's brain-snap decision to unbuckle his seatbelt as he travelled through pit lane.

Since combining in 2012, the pair have won twice at the venue, in 2013 and 2014.

Friday's practice results suggest a third win could be on the cards.

Dumbrell burst out of the blocks to smash the practice lap record - set by Whincup in 2014 - by covering the circuit in 1:07.8467.

Whincup followed that with a 1:07.9688 lap, meaning they were the only drivers to break the 68-second barrier.

Dumbrell was pleased with the results but would plainly swap Friday's efforts for a decent qualifying run.

"The track is great today but it will be diabolical tomorrow. It looks like it's going to be wet and get worse through the day. We'll just put our heads down," he said.

The 500km classic is the first of three defining races for the Supercars series with next month's Bathurst 1000 and Gold Coast 600 to follow.

In the drivers' championship, Scott McLaughlin sits just 19 points behind leader Shane van Gisbergen, with a major gulf to Whincup in third.

McLaughlin is once again teaming with Alex Premat, who came in seventh and 23rd during the two co-driver sessions.

Supercars debutant Earl Bamber, who partners fellow Kiwi van Gisbergen, improved from 14th to second in the ZB Commodore.

Van Gisbergen was just a tenth of a second behind Whincup, while McLaughlin was slightly further back.

Qualifying for the long-distance race takes place across three sessions on Saturday - Sunday's race starts at 1.20pm.

AAP