Bad surface created

The solid in the pic was trimmed with other solid. But as the result, the lines got clung on the surface. Why did that happen and how do i solve such problems?

If this is a planar surface, and of the rest of the object is a valid solid (with no glitches), then just delete the bad surface, select and join the rest, and type Cap while the object is selected.

Cap should - if the opening is planar - simply close the opening left behind after deleting the bad surface.

// Rolf

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That’s an interesting question. Can you repeat the process? If so, please post a file and instructions.

Try exploding the solid, selecting all the surfaces, run the RebuildEdges command then join them all back together.

This is most likely caused by the cutting object you used which may have some overlapping surfaces or be just out of tolerance. The object is simple enough that I would just trace a closed polyline around one side (to have absolutely clean geometry) and then extrude it to the depth - then delete the original.