It's warm weather that causes excess mastic to ooze out.

It's not difficult to remove it - use a blunt plastic scraper to get the worst off (an old bank card is ideal), followed by a rag and some white spirit.

It's a faff, and it's messy, but it will come off. If you've waxed the paintwork you'll need to redo it afterwards.

You can take some small consolation from the oozing - if there wasn't enough in the joints, there'd be no excess to come out.

Too much is better than not enough.

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Removal is as Pepe says above.
The excess mastic was so bad on our new van collected in Feb that it took me several hours to address. On one side it actually filled the gutter, blocking it totally. On the other side so much had squeezed out that the leaves attached to it had blocked the gutter. I removed the excess with old credit card being careful not to pull it out. In other areas it seemed to be the opposite and was lacking mastic. After a good clean with white spirit I masked the area off and ran a bead of white sikaflex into the joint edge.
The corner drain holes were also next to useless as the swarf from drilling had not been removed.Careful use of a needle file sorted that one out.
Disco lights also prevail and the van is due to have these replaced along with three top locker doors.

A shame really as the rest seems to be well screwed together.

Colin