pastel Chalk panel lines wash

smeagol is right.  If oyu are just going for panel line accenting it needs to be done over a gloss surface.  I use pastel washes al the time.  If you do it over flat paint it kind of "stains" the paint.  It takes soem effort with a stiff brush to remove it.  This is a useful technique though for wheels wells and inside open panels etc. where oyu kind of flood the area with the pastel wash, wick some of it away with a cotton swab then use a stiff brush to remove more as needed once it is dry.

Gloss is better.  This image (from cody's coop, for decals) shows why.  Flat surfaces arent smooth, there rough and leave places for things to hide, little gaps and such, where gloss is a smooth surface.  The flat surface would be harder to remove the pastel from, making the line to dark at times, inconsistant.  I would suspect atleast, this is my experiance with washes.