Two weeks ago, on a rooftop 11 stories above São Paulo, a popular Brazilian street artist, Mundano, sat on an overturned bucket, mixing water, varnish and ash collected from fires that had ripped through a Brazilian rainforest to create a palette of gray tones.

Over the ledge awaited a newly whitewashed, 15,000-square-foot wall of an elegant apartment building in plain view of the buses and cars heading down a main artery leading to the city center.

That evening, he and five assistant artists would start painting a massive mural of an Indigenous leader, Alessandra Korap, in a scorched Amazonian landscape, holding up a sign urging Cargill, the Minnesota-based agricultural giant, to rid its supply chain of crops grown on recently deforested land.

The project is a collaboration with the conservation nonprofit Stand.Earth, which is funding the mural as part of a campaign targeting Cargill.

The final result is to be officially unveiled on Wednesday, though it is hardly a secret to the supermarket shoppers, passers-by and those who work in the small shops that surround a parking lot below the mural.

“I’m already tired and we haven’t started yet,” said Mundano, whose (rarely mentioned) first name is Thiago.