If you're planning to use a panel line wash (something like Tamiya panel line accent color for example) than putting a gloss clear coat over the paint is a good idea for two reasons:
- The washes flow better over a gloss surface.
- Such washes are mostly enamel based (acrylic ones do exist, but aren't as common), and some enamel thinners can dissolve acrylic paint. An extra coat of paint theoretically gives you some extra protection. I've never had that happen to me though, so I can't say how big of a deal it is in practice.
An additional important note is that for an enamel wash, the clear coat needs to be acrylic (or lacquer, I think). With washes, you always need to do some clean-up using enamel thinner, and doing that over an enamel clear coat would also likely wipe the clear coat away.
I don't personally know how this applies to panel lining with a marker. I would imagine clean-up would be easier on a glossy surface so a gloss coat might help. I don't know how much it matters though.
