To simulate butt joints, I use 1/64" chart tape. After glassing and once the surface is smooth, I layout the lines from 3 views in pencil. I then run the tape over the lines. This can be a bit tricky with such thin tape, it wants to curl and the adhesive side tends to flip up. Once the tape is all on, I use a high build sandable primer and give the tape several coats, feathering the primer out as you get further from the panel edges. I use about 3 or 4 coats. Once the primer is dry, pull up the tape leaving a trench in the primer which simulates the joint. Often the adhesive will stay in the trench. Once the primer is fully cured, you can go back with a solvent and a tooth brush and scrub it out. Next give a light sanding with 320 or finer grit paper to smooth any rough edges.
for lap joints, where one panel is raised, I use masking tape to tape over the lower panel. Once again I spray the joint with high build primer feathered out toward the center of the panel. Once dry, remove the masking tape and you are left with a raised edge. Lightly sand and you are now ready for rivet detailing and painting.

Scott