From Reggie on Cruising World message board:
As I see it you have two options.
Quick and dirty - Buy yourself a quart of Interlux Brightside.
Dewax the surface with Interlux 202 solvent wash or equivalent solvent.
Mask the area off and sand the old stripe smooth with 220 grit paper.
Clean up and tack the surface and remask it with fineline tape
then apply two coats of Brightside using a good quality brush.
Hard core Yachty way - Dewax the surface with Awl-Prep Plus wax
and grease remover. Mask off the area and sand it smooth ending
with 180 grit. Fair out any imperfections with Awl-Fair L.W. fairing compound.
Sand smooth to 220 grit, tack off, remask with fine line tape and
prime with Awlgrip high-build epoxy primer. Sand smooth
with 220-320 grit paper. Clean the area up, remask with fine
line tape, tack the surface off and then roll and tip on two
coats of Awlgrip 2-part linear polyurethane.
The Brightsides will give you a finish that will last 3-4 years
before it starts to look rough. The Awlgrip finish will last 10
years or more and will look great.
From "Offshore Cruising Encyclopedia" by Steve and Linda Dashew:
If you paint a straight bootstripe, it will look hogged; like it is drooping at the ends.
... If the stripe is 3" thick at the middle, I'd make it 6" at the bow and 4.5/5" at the stern.