Alonso's problems started when he ran wide onto the gravel at the final corner as he started his first lap in Q1 in Barcelona.
He recovered to get out of that session in 12th and then took eighth in Q2. With the car in a better state for Q3, Alonso believed he could still have made the front row, but a mistake on his final lap consigned him to ninth.
The team managed to gradually repair the floor during the gaps between sessions, but Alonso faced the additional challenge of not knowing quite how the car would behave each time he went out.
He has subsequently gained a grid spot after Pierre Gasly was handed a pair of grid penalties for impeding - totalling six positions.
"It was opening the first lap in Q1," said performance director Tom McCullough. "And he had a snap on the way in, whether it was a little damp patch, or probably pushing a bit hard. So it was just more of a low grip snap, the track not quite in good enough condition.
"When you fly into the gravel trap at that speed, it's just like sandblasting the car and the underneath, the strakes and everything got a real beating.
"It ripped off some of the vanes and winglets underneath as well. So yeah, we were definitely quite sub-optimal with the car."
While the crew in the garage tried to improve the car for each session engineers back at the team's base monitored the changing balance.
Source: Autosport