Patric Niederhauser threw in the quickest time of the day in GP3 testing before the heavens opened on the Barcelona track.
Niederhauser led teammate Alex Fontana in a Jenzer team 1-2, four tenths of a second apart, as the former's lap of 1m33.750s stood as the day's fastest.
Manor duo Tio Ellinas and Nick Cassidy were next up, followed by Giovanni Venturini for Trident. Thereafter the mission was simply staying on the road, with many drivers aquaplaning off in the extremely tough conditions.
Jack Harvey, Cassidy, Carmen Jorda, Emanuele Zonzini, Josh Webster, Facu Regalia, Samin Gomez, Melville McKee (in for Lewis Williamson at Bamboo), Tuesday pacesetter Conor Daly and Carlos Sainz Jr. all had incidents, with Sainz damaging his car heavily at Turn 3.
Times meant little, but Aaro Vainio emerged quickest for new squad Koiranen by 1.6sec over Harvey.
GP3 testing resumes at Silverstone next month.