A thin wall is the print fault that looks fine in Blender. The mesh is manifold, the scale is right, the render is clean. Then the slicer preview shows a gap where the rim of the cup should be, or the part prints and the edge snaps off in your hand. The nozzle simply could not lay down a line that thin.
You can catch this in Blender in about a minute, before you export anything.
Know your number first
Before you open the check, decide the thinnest wall your printer can actually make. On an FDM printer with a 0.4 mm nozzle, a common rule of thumb is two lines, so about 0.8 mm. If the part has to take any load, 1.2 mm is a safer floor. Resin printers can go thinner, but check your printer and resin rather than guessing. Write the number down. That is the value you will type in a moment.
Get the scale honest
The thickness check measures the mesh in scene units. If those are wrong, the check is wrong.
Select the object and press Ctrl+A, then Scale, so the object scale reads 1.0. Press N and look at Dimensions in the Item tab. A 60 mm part should read 60 mm. If it reads 60 m, sort the scene units out first. The scale and STL guide on the site covers that step.
Run the thickness check
Enable the 3D Print Toolbox if it is not on already. In Blender 4.2 and newer it comes from the extensions site. Go to Edit, Preferences, Get Extensions, search for 3D Print Toolbox, and install it.
Press N in the viewport and open the 3D-Print tab. In the Checks section, find Thickness. Type your number into the value beside it, for example 0.8 mm. Click Thickness.
The result appears in the panel as a count of thin faces. Click that result. Blender goes into Edit Mode with those faces already selected. Now you can see exactly where the problem is instead of hunting for it.
Fix only what it selected
Leave the selection as it is. What you do next depends on what the faces are.
If the whole object is an open shell, like a plane bent into a cup with no inside, the fix is a Solidify modifier. Add it, set Thickness to at least your number, and apply it before export.
If the object is already a solid and only one area is thin, like a rim or a fin, press Alt+S to shrink or fatten the selected faces along their normals. Move the mouse to push them out a little, then click. If the faces are on both sides of the thin part, push them in opposite directions so the wall gets thicker, not just moved.
Run the Thickness check again. Repeat until the count is zero.
Check it once more in the slicer
Export the STL and open it in your slicer. Look at the layer preview at the thin area, not just the solid view. Every wall should show at least two lines. If one still shows a single line or a gap, go back to Blender, raise the thickness value slightly, and run the check again.
While you have the 3D-Print panel open, the Overhang check in the same section does the same job for steep undersides. The 2024 printing overview on the site goes through overhangs and supports.
Print it right the first time
The full workflow, from modelling for FDM and MSLA to export and slicing, is in my 3D Printing from Zero to Hero in Blender course on Udemy:
