Friday practice for this weekend's Rabble.Club Sandown 500 has been completed, with both Red Bull cars looking like the cars to beat early on. 

The Red Bull Holden Racing Team showcased leading pace across the first two practice sessions. Jamie Whincup's co-driver Paul Dumbrell dominated the opening session by resetting the Sandown practice record (1:07.8467), which was then followed by Whincup topping session two. 

The Red Bull train was pipped in session three by impressive Supercars rookie Will Brown. But two-time Le Mans winner and Supercars debutante Earl Bamber still clocked the second fastest time of the session. 

Conversely, Scott McLaughlin and co-driver Alex Premat both struggled. Seventh in the opening test was followed by 11th and 23rd — engineer Ludo Lacroix noting a lack of straight-line speed along Sandown's long straights being part of the problem. 

Click here for the full grid of 'retro round' special Sandown 500 liveries

Dumbrell's extreme pace was chalked up to increased grip at the iconic venue from a unique filler compound used to repair various points on the track surface. He led Nissan's Dean Fiore, Jack Perkins, Alex Rullo, and Jonathon Webb in first practice. On debut, Bamber was the lead Kiwi co-driver in 14th, with Chris Pither a few spots back in 17th. 

Whincup's showing in second practice saw him lead teammate van Gisbergen, James Courtney, Garth Tander, and Craig Lowndes. Andre Heimgartner and Fabian Coulthard also cracked the top 10 in seventh and eighth, while McLaughlin could only muster 11th — even with the aid of a 'green' set of tyres. Richie Stanaway completed the Kiwi line-up, limping to 24th after a string of off-track excursions.

Bamber's improvements continued in the co-driver-only practice three. He led briefly until being pipped by Brown on a 1:08.6230. Richards, Perkins, and Luff completed the top five, with Dumbrell in sixth.

Pither wound up in 24th, one spot behind McLaughlin's co-driver Premat who continued to struggle to put a competitive time on the board. 

Whincup's session-two pace (1:07.9688) slower than Dumbrell's in session one, and it's forecast that the grip gains from the revised surface are going to drop off further over time. 

Speaking of forecast, it's expected that the weather is going to include plenty of rain over the next two days — with the potential of hail being reported. Sandown is known for its wet races, with 2016's thriller (won by Tander and Luff) being the most recent. 

The sessions themselves were relatively free of incident. The largest of them happened to Tickford Racing's Chaz Mostert at turn four in practice two. After straddling the kerb on the outside of corner entry, the 2014 Bathurst 1000 winner spun backwards into the tyre wall at a high enough speed to fracture the boot-lid and cause lengthy repairs between track sessions. 

On his way to topping practice three, Brown also went off at turn six through the gravel trap — laying down a thick dust rooster tail to rejoin the track. 

Tomorrow's Supercars schedule kicks off with a fourth practice session at 12.05pm NZST. Then there is a minor qualifying session at 1.25pm, which will set the order for the first 'race for the grid' sprint in the afternoon. 

Those sprints begin with a short, sharp 20-lap race for co-drivers only at 3.15pm. Another 20-lap race follows at 5.50pm for the 'lead' drivers, and the finishing order of that race will define Sunday's Sandown 500 grid.