Jean-Eric Vergne reckons the new Pirelli tyres resemble "cauliflowers" after just one lap given the severe graining.
Vergne spent the past two days putting in the laps at the Circuit de Catalunya, developing Toro Rosso's STR6 as well as learning about the new Pirelli rubber.
The compounds have been heavily criticised this week with the drivers complaining of extreme degradation.
Giving his verdict, Vergne said: "They were looking like cauliflowers.
"I used this expression in French quite a lot when I was in karting... it's really tricky. I have only one lap, then you have a lot of graining, so it's difficult."
The Frenchman, though, is hoping the degradation will play into his hands, especially in qualifying.
"If there is one thing I like about these tyres is that it's a one lap tyre. I think that's really nice, you have one lap and you really have to go for it and that's it," he told ESPN.
"It suits my style, because I'm quite aggressive. I wouldn't say I'm aggressive all the time, but just in the way of my approach.
"I mean outside the track, on the track and in the race I try to have a really smooth driving style to try and help the race, but when it needs to be pushed, I'm there."