Jenson Button has said there are positives and negatives with the upgrades McLaren has brought for the Spanish Grand Prix this weekend.

McLaren has brought a number of modifications to Barcelona's Circuit de Catalunya for the fifth round in the 2013 F1 World Championship in a bid to tame the troublesome MP4-28 and bounce back from a rather woeful start to the season.

However, despite that, Button remained well down the times in practice on Friday and following the second practice session was only 12th quickest, well off the pace.

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"As with any test of an upgrade package, there are positives and negatives to take away from today. There's now a better understanding of the correlation between the track and the wind tunnel. That's positive for the future, but of course we'd always like more. So I think we've moved forwards, but I think the others have too," Button explained.

"We were limited in terms of the amount of running we could this morning [due to the rain], so there's a lot of data to sift through and plenty of things to change for tomorrow.

"We need to look at those elements that are potentially working and those that are not helping the overall car balance. We need to focus on a couple of areas - but, hopefully, we can improve them a bit by tomorrow."

Team-mate Sergio Perez, who finished the day just behind Button in 13th, also didn't seem overly impressed with the changes to the MP4-28.

"Today was basically all about the afternoon session, and collecting as much data as we could from some very intense running. It possibly wasn't the day we were hoping to have, but we collected a lot of data for the engineers - that's a positive," he added.

"It was difficult to draw any clear conclusions from today, but the main positive we can take away is that we have a lot of data to feed into our simulation software in order to improve our understanding of the correlation. We've made a slight improvement.

"Once we get a better grip on our understanding of the correlation, I think we'll start making bigger steps. Over the next few races, hopefully we can make more progress."