I have not painted Jasta 5:red rims. But from my experience with PC10 returned to the bottom of wings and stabilisers - I find it easier to airbrush the interior, mask it and then airbrush the rim. Before any assembly, when the parts are free to manipulate I use them to draw the outline of the surface on a paper. Then I draw the line inside the shape. Later this template is used to prepare the masks from ordinary Tamiya masking tape. You only have to be careful, that you get a mirror image of the shape. But it is not a problem with Jasta 5 tail as you need the masks for both sides.