I have to say, I much prefer using a wash to using Gundam markers. This might not be the most popular opinion, because it takes a lot of precision not to screw it up with smears. This is just a matter of preference and convenience. I often feel that using markers results in lines that are a bit to bold looking. Especially if you're using a black Gundam marker.
Also it just seems to me like you can totally tell it was scribbled in. I mean, it's a method many prefer because of the convenience, and from a zoomed out pose it looks just fine. I'm just really anal about detail so it bothers me a bit. Not to mention, there's just something really satisfying about dabbing a bit of paint on something and watching all the panel lines fill up nicely. It's like something out of an anime where a mecha gradually lights up or something. So instead of markers, I often make a wash of black enamel paint and thinner. I use about 3 drops of paint and 2 drops of thinner, but you have to play around with your own paint to get the best ratio. Consistency wise, you'll want to aim for something that rolls down the dish but not too much. Something a bit like this:
The trick is finding a really fine point brush. With one of those, you'll reduce the chance of smearing paint where you don't want to. After the paint dries (I try to give it a good 30 to 45 minutes), you can clean up using a Q-tip lightly moistened with thinner. Try not to put too much pressure on it though. You don't want the thinner to leak down into the cracks and ruin what you've just done. After it's all said and done, you should be left with some pretty sweet, clean looking panel lines! Until next time, ciao!