Waterlines

There are plenty of thin tapes available. What width do you need?

I use your method normally.

I first learned it painting "NO STEP" lines on aircraft wings.

3M makes some vinyl masking tapes in various narrow widths.   This tape is used in the auto painting industry to lay down lines which follow the various compound curves on a car body.  The tape is also used to lay out scallops and flames on custom car paint jobs.    It is not pin striping tape which you may find at the local Pep Boys/Trak Auto/Auto Zone/etc.  Check the local auto paint supply house in the yellow pages.    I use some which is about 1/8 inch wide

I make a decal.  Inkjet decal paper has been perfected and it is now  practical to make your own inkjet decals. I have made both plain black "boot topping" water lines, a single black stripe, and ones with a finer white stripe at the edge of the black stripe.  The latter is a bit harder to do, the single black stripe is easy.  Because my decal paper is only eleven inches long, I sometimes have to make two or three sections for each side of the model, but this is no problem.  BTW, the decal stripe will also cover slight masking defects in the underlying paint :-)

You can also get thin decal stripes from Microscale in various colors, but they are expensive.

The line will probably end up being about 3/32" wide.  Making my own decals for this is something I haven't thought of.  I'll have to look into that.  Thanks to everyone for their comments!

Remember, there is a Fine Scale forum on decals, near the end of the list of forums.  Making inkjet decals is a frequent thread, so if you have any trouble, pop over there and post.

A fine point you may or may not care about:

The boot topping stripe along a waterline may not be the same width over the entire length.  This is because it usually is from a certain draft level to a higher ( or lower ) draft.  If the hull is not more or less vertical, as down under the stern counter, the stripe will be wider.  If there is a lot of flare to the hull it could be noticeably wider, though as viewed directly from the water level, it looks the same all along the length.

Fred

You may want to look into this parallel cutter:

www.micromark.com/parallel-cutter,8715.html

I'm a big fan of Aizu tape, which comes in rolls of various thicknesses. When I have long hulls with flare (I.e. not a consistent width for the whole length) I like to use a strip or two of the Aizu tape. I usually mask down one side, cut, then mask the other side.

I used the method described above with 1/8" masking tape (you can buy it down to 1/64" if needed)

sprayed black for lineapplied masking tape

sprayed red for lower hull

 after taping off lower hull, sprayed gray for upper hull and then removed all tape

(...and the finished model)   Hope that helps Tim