Turned out well in the end.
"Maskol" and similar masking fluids, are really intended for clear parts, where it is mainly used as a "filler", with tape at the edges. Or, on canopies for instance, it can be applied overall, allowed to set, and then the frame areas can be exposed by cutting the dried fluid where needed, ad peeling off, using a new, or sharp scalpel blade to do the cutting..