Masking

Is the curve single curvature or double (complex) curvature?  Masking tape bends around curves pretty easily.  And, if it is very narrow, it will conform even to double curvature.  I would use a thin tape, like the Tamiya stuff.

The problem will be cutting narrow but uniform widths (I assume the space between the lines is narrow also).  Cut with a stainless straight rule.  The masking tape is reasonably cheap, so even if you have to discard the majority of the pieces you cut, you aren't wasting that much.  Apply the tape to a piece of glass and then slice the narrow strips.  1/16 inch width is available at many hobby shops- you may not even have to slice that stuff.

To mask a large nose area that went up one side of the fuselage, across the cowl and down the other side in a swooping arc, I used a French curve and drew some trial and error patterns on paper.  When I finally got one that was close enough, I transferred the pattern to a large piece of frisket, cut it out and stuck in on as a mask.  Worked fine!

Thanks a lot for your answers! I´m actually trying Don´s method but, maybe I was using tape stripes which were too wide, I´ll try cutting narrower ones. I used Cadet´s method in my last project, but in this case, as there are three very thin lines to paint, the tape "mask" becomes too fragile to handle.