I'm printing a skirt with 0 offset (should touch part). The part has an undercut at the table surface. Note that the skirt stops at the major diameter above the 0 layer, rather han coming all the way to the perimeter at layer 0.
don, the skirt doesnt necessarily follow the first layer. it follows the overall top view of your model. so looking straight down on you model it would draw the skirt around that shape.
Here's how it works for a skirt offset of zero
- If "Generate support" is disabled, then it does exactly what you want. The skirt would touch the edges of the bottom layer creating what people seem to call a "brim"
- If generate support is enabled then the outline for the skirt does what jimc mentioned. It uses the silhouette of the part, which may or may not match the bottom layer outline. It depends on the geometry of your part.
I think this makes sense, because if you are using support material, you don't want the skirt interfering with the base layer of your supports. That first layer for your support structures is really important, and you don't want the skirt messing it up.