Is enamel paint supposed to be this thin???

You should also use primer. That'll take color coats better than bare plastic.

I use enamel a lot and no, it should not be that thin. As Mick says, looks likes it needs a proper stir, shaking the tin just is not enough.

Hi Nick.

No, I use Humbrol and I will always have to thin it a bit to brush paint. Looks like it needs a very good stir with something metal and make sure you have ALL the paint from the bottom, Humbrol can settle very hard on the bottom.

Mick.