Stephen,
Depending upon the application, I almost always use the decals provided in the kit. The reason they look washed out is because the background your putting them over is too dark.
Lets use the Gulf GT40 picture as an example… I would probably paint the blue and orange but then use the kit decals over the painted stripe. That way if the decals don’t totally cover or leave a peeking edge somewhere it’s not a total loss. Any breaks in the black line can be touched up with a tiny brush or technical drawing pen and India ink.
Cover all decals with a couple of clear coats to smooth out the overall sheen, rub out the clear and your done!
As for the question about painting of a black line… The aforementioned technical drawing pen works great and comes in a variety of line widths from “0000” to … well, really wide!
You could use that to draw black lines freehand or with a ruler.


The pen on the left is a “00” Rapidograph/Koh-I-noor technical pen. They are pretty expensive and this one is no longer available. (My pens usually last me 10 to 15 years but I use them every day! If left unused for longer than a couple of weeks they will dry up.)
The nib on the right however, is neat in that it’s adjustable. Turn the thumbscrew and the tips narrow. Ink is held between the two tips through surface tension. Paint can also be used with this method but if paint is thinned out enough to work in this particular device it may not cover.
The nibs are also very easy to clean and relatively cheap to purchase… if you can find them!
At any rate… If you use either of these methods then you need to prep the surface very carefully first! Once the base color is totally dry, wet sand the whole car and get the painted stripe edge knocked down. Plus sanding puts a bit of a “tooth” on the finish so the ink will stick. Wipe dry with a clean towel. Wash your hands really good! (This cannot be stressed enough because the ink will not dry or stick where body oils are deposited) Use the ruler to guide your pen strokes and try to only make one pass with the pen. Let each pen stroke dry totally. If it’s not dry it will smear when you drag the ruler through it! (and you WILL do this…)
For curves, use a French curve but don’t try to freehand it.
This is the ink I use:

Once dry India ink is permanent but it can be wiped off with a damp cloth before dry if you screw it up. I think rubbing alcohol will take the dry lines off if your unhappy but India ink formulas differ by brand. Always clear coat over any ink lines!
If you have decals that are to go on too…. Be very careful, as the decal set, if used, will wash away the ink unless you seal it first. Better is to put the decals on first and draw the lines after the decals are totally dry and you have wiped the body down.
I guess that’s it, Good Luck!
8)