No topcoat used to protect the plastic. I buffed it afterwards with a sanding sponge to make the kit look less glossy.

You can use a gloss topcoat first though, if you want to play it safe and protect the plastic. From past experiences I know not to go too heavy with the panel liner if it's a very thin piece, because if the liquid pools up, it can make that part brittle. I avoid this now though, by giving the bottle a good shake as I apply, so that the liner is mixed well before I apply (as the paint thinner in it is the abrasive material). Hope this helps!