So you basically invented accidental stone stamping with a side of burlap, congrats on the free upgrade. But yeah, that blotching is the burlap's fault, not yours. Where the folds sat, the fabric stayed wetter and pressed tighter against the cream, so those spots cured slower and darker than the flat areas. Phoenix heat makes it worse because the exposed high spots dry out fast while the fold lines keep sweating. If you want even color next time, ditch the burlap and go with a single layer of wet blankets or plastic sheeting pulled tight so there are no overlap lines. Or just tell every customer it's a custom finish and charge extra, since yours already thought it was stone.

Your buddy's wrong, that's literally how bakers measure flour.