Mercedes ended the second test the way they started it as Lewis Hamilton put the W04 on top of the times sheets.
But showers in the morning, more rain in the afternoon and a string of red flags meant a disrupted day for all the teams at the Circuit de Catalunya.
Hamilton set his quickest time when the track was at it driest during the morning running. He stopped on track with a technical problem during the afternoon.
This was one of eight red flags which hampered teams’ efforts to conduct meaningful running during the test. The first came just nine minutes into the session when Felipe Massa went off at turn four.
Giedo van der Garde caused the next when he went off at the final corner. The Caterham came to a stop of a second time later in the session.
Another driver who triggered a pair of red flags for Esteban Gutierrez, who went off at least twice in the tricky conditions, including another occasion when he was able to keep going.
Another red flag was caused by Jean -Eric Vergne, who was also a victim of the conditions, spinning at turn five.
The final red flag came a few minutes before the end of the session and brought the day to an early close. Jules Bianchi, who took over from 2013 seat rival Adrian Sutil in the Force India, stopped at the pit lane exit.
Mark Webber also came to a stop in the Red Bull at Campsa.
The poor conditions and persistent stoppages led Williams to ignore timed runs and perform a series of pit stop practices instead with Valtteri Bottas and Pastor Maldonado.
The final four days of pre-season testing will begin on Thursday at the Circuit de Catalunya.
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