... STL model because it is not "watertight", which means (I think) that there are some gaps in the surface. Meshlab is a pretty good tool for identifying and repairing any gaps. So, as far as I know, no filler is required. STL only defines a surface (I think, maybe). Andy posted: 22 Jul 2018 04:07 from: Martin Wynne Hi Andy, Some progress, but still a long way to go. As you can see, the rail is hollow, 2_212217_040000003.png because it's intended to connect to the next template, so the rail end is never visible. I have made no attempt to do the top corner radius on the rail head or the web fillet radii, life is just too short and it would massively increase the file size. See later. Where the rail end is visible on check and wing rails I have added end caps (as on the printed templates): 2_212217_040000004.png But the rail is still hollow. Of course for most 3D printing the rails would be omitted ...