To do this, you have to go to the "List of welded parts" folder, right-click, update and check "automatic".
Then in the properties of the article of the welded parts that are created underneath, all the properties are filled in including that of the surface of the sheet metal sidewall.
These properties are recoverable on the sheet metal drawing, right click on the unfolded view and choose "Annotations" and "Welded Parts List Properties"!