Mercedes suffered setbacks on both of the first two days of running in Bahrain, leaving the team at the bottom of the mileage chart heading into the final day.
Bottas was able to set the fastest time of testing so far on Saturday evening, giving Mercedes a boost after a difficult day that saw Lewis Hamilton spin into the gravel and cause a red flag in the morning.
Bottas said that although windy conditions in Bahrain were not making things easy in the car, he could notice the rear end of the W12 was tricky to control at times.
“It’s a bit more windy conditions than we’ve had normally at the race,” Bottas said.
“But still, I would say one of the big issues with the car is the rear end. It’s quite snappy and it’s quite unforgiving.
“That topped up with the new tyres, that they feel quite sensitive to the sliding, it’s not that easy.
“We are kind of trying to calm the car down a bit and that way trying to get some more pace.”
All teams have been working to reclaim downforce lost over the winter after tighter regulations were introduced for the floor and diffuser designs, making it a key battleground heading into the new season.
Mercedes was notably secretive about the design of its floor for 2021 prior to the start of testing, and also avoided disclosing where it had spent its development tokens on the W12 car.
Bottas lost the majority of his first morning in the car to a gearbox issue on Friday, and will complete his final outing on Sunday morning before handing over to Hamilton in the late session.
The Finn was able to take top spot on Saturday evening after a run on the C5 tyre, but said he wouldn’t be “jumping around and being happy to be first” given it is only testing.
“As always we really just focused on getting through the programme and maximising the learning and not really looking at lap times,” Bottas said.
“We have no idea what others are testing, and what’s going [on with] fuel loads and engine modes, so it’s hard to read yet.
“Hopefully, by end of tomorrow we could get a bit of an idea from long runs where we are, but still it’s only testing.”