Ah, panel lines. Every model builder has a different method to tackle them and there really isn't a right or wrong, just what works for you.
I've tried a couple methods over the years before finding what gave me the best result, and that is to paint the panel lines before applying primer or paint coat. The method is explained in detail on
Model Cars Magazine Forum. I like this method since you don't have to worry about being too neat and tidy when applying paint into the panel lines. In theory, putting primer and paint over top decreases the panel line thickness and makes it look a bit more realistic.
Pic attached is my Supra done using this technique.