Unless you're printing in the Spiral vase mode, each perimeter loop has to start and end somewhere. Furthermore, the printer has to stop extruding for a brief moment when the print head moves up to the next layer. This start/endpoint creates a potentially visible vertical seam on the side of the object. It's also commonly referred to as zits, layer seams or scars.
You can set the Seam position in the Print Settings to try and hide the seam in a corner, randomize the seam position or align it with the back of the model.
The seam painting tool provides more detailed control over the seam placement. The tool is accessible from the left toolbar and it is only shown in Advanced and Expert modes. After clicking on the icon, the user can paint 'Seam enforcers' or 'Seam blockers' on the model in a similar way to Paint-on supports.
Left-mouse button - Enforce seam
Right-mouse button - Block seam
Shift + Left-mouse button - Erase selection
Alt + Mousewheel- Change brush size
If there is a seam enforcer, it always places seams into the enforcer area.
If there is a blocker, its area is excluded from possible seam candidates.
The Nearest, Rear and Random Seam position options are still respected. When the Aligned option is active and enforcers are used, the seam is placed in the middle of the enforcer area. This allows drawing smooth seams over the model.
If a layer has multiple seam enforcers - WILL BE UPDATED SOON
Spherical cursor (default)
Paints everything inside the sphere regardless of whether it is visible or not from the current view. The preferred method for most cases.
Circular cursor
Paints all visible places inside the circle from the current view, possibly leaving unpainted areas, but never painting behind corners.
You can use the clipping plane tool to hide a part of the object, so that difficult to reach areas can be easily painted. Move the slider from left to right to move the clipping plane.
The Reset direction button will align the clipping plane tool parallel to the current camera view. E.g. if you want to have the clipping plane moving up-down, look at the model from the top (Top view 1 ) and hit the Reset direction button. The button appears when you move the slider to any non-zero value.