Model Cars and vehicles

I have the luxury of a "cutting laser" to cut routing templates, however, they could probably made with a scrollsaw/jigsaw (carefully).
My Hummer build should detail how I do it... though Master  Yoda  Dutchy could provide better insight... though he now has a laser as well (sorry for the reveal Dutchman).

For something like that I'd instantly turn to the laser for etching.
If not, you could use an ink marker. I've used rapidograph pens (drafting tools) before, india ink and precise lines/widths.
With the pens though you'd need to pre-seal the wood so it doesn't suck up the ink and bleed, then top coat to preserve it.
Another option is decals.

Thanks Guys, I have been considering a laser , I will need to do some research.

Naturally it will all depend on you cache of shekels, and with SWMBO's approval, may I suggest you hook up with a laser that will cut at least a minimum of 3mm MDF (or at a pinch 1/4").  
When push comes to shove, you can use the cut MDF (may need to laminate 2 layers of 3mm to get contact surface) as routing templates... engraving will then become a bonus.

In the past, I have created several blogs about making kerfs, using both hand and machine tools. However, there are so many blogs that I no longer know where they are located. I definitely want to add some new articles to my How to page soon, but at the moment I am busy with my new laser.

Speaking of lasers, you should realize that if you use them to make kerfs, they will turn black. Although that is an accent, you have to want that.

I'll check my LJ history in a moment to see if I can find something for you.

I do remember, however, that I made most of the kerfs (the straight ones) for the SUV on the saw machine. I have a saw blade of 150x1.5 x arbor hole. The bevel groove was made with a V gauge.

But you can get a lot of enjoyment out of a laser. Just think of the inscriptions you can create on your models. A laser is also suitable for tires with text, and ...... also for making templates if you buy one above effective 20/30 Watt.

I thought I had made the notches on the SUV with the saw. But that isn't  true. Have you already checked out my blog series about the SUV here on Craftisian, because that is where you can find the blog:  Part two about making the kerfs.

Just to expand on  using laser cut templates... though I had better detail from the past, but not having any, I had to reinvent my lies (and fake  news  photos.).

Lucky I've kept all my past model templates,

I didn't remember making so many templates for my Hummer,

though that was back in 2015 when I still had an abundance of MDF trees in my back yard.

I made up templates to hold the solid parts that were made out of Maple and were to be "door kerfed" using my Hummer's Sketchup model,

Using this template to hold the solid part,

I overlaid it with the kerf template using alignment dowels in the corners and used one of these following units

,
in a small router/trimmer, to rout the kerf using the overlaid template,

The customised Foredom

that I quoted in my Hummer post, was incorrect and was used in my wheel treading jig,

I’ve only done straight kerf lines on my models so this probably won’t help you much. I use a 1/16”  bit on my router table. Determine where the line will be then flip the piece over and guide it across the bit by sliding it along the fence. 

Old school. and dependent on size of the model. Don't forget a saw kerf from a NON through cut. about an 1/8" space from a TS blade. Bandsaw kerfs are a lot less. 

Like Duck showed, a few templates were the pre CNC or Laser way to get it done. Since the increase of CNC for woodworking an entire line of straight cutting bits of varied sizes. If you go "twisted" remember you want either a downcut spiral, or compression for a smooth surface. If seeing the bottom of the cut is normal, invest in compression bits, they give a finely cut top, and bottom in one pass. IOW NO splintering. 

best of luck, no matter what method you use.