Michael, why when I select all surfaces by select all or by selection rectangle then join them I get joined surface but when i select all surfaces one by one and click join i get solid. I just wanted to make solid and meat this problem.

Hi mir4ea - you've got one surface duplicated on top of itself there. If you delete the duplicate you can then do a select all and do the join.

It doesn't work to join 2 surfaces that are stacked up directly on top of each other, the joiner then tries to glue stuff between those 2 surfaces along with the rest of them.

To delete the duplicate, click on each surface you can see and then do an invert selection and then delete. The duplicate is one of the longer side wall ones, the one on the right-hand side of the object after you open the file you posted.

- Michael

Thank you, I meat this problem sometimes, but have no idea how to solve it, thanx