A pin wash is more detailing rather than weathering and its main purpose is to create depth. Typically you'd apply your base layers of paint i.e primer then the base colour, possibly apply a filter then once cured apply either a thin layer of satin or gloss varnish to seal it. Don't use matte because it will cause problems. The next step is the pin wash, where you'd typically use either thinned enamels or thinned oils. The varnish stops the base layer absorbing the thinners/oil and allows capillary action and easy cleaning. You take a thin brush, apply your pin colour, typically a darker colour of your base and apply it to details and panel lines. The reason to do it with enamels or oils over a varnish, other than the aforementioned reasons is it's extremely easy to clean up where you don't want the wash such as raised surfaces I.e. rivets or welds. Both perfect for pin wash bit also highlighting on the exposed surface. There are acrylic washes but honestly, they are no where near as easy to use it get the same effect. Some people will so several pin washes at different stages i.e. a dusty tank with streaks of oil, diesel etc. You'll do your dusting coats and other weathering and then pin wash again with a darker dust colour to create depth in places where it's needed.
On YouTube, Google Mig Jiminez. He does some good videos on these techniques. For plastic models his products are also some of the best.
Graphite is good stuff for metal detail. Despite all the products that get sold, you can do almost anything with oil paints, thinners, graphite and a a good set of acrylics that can be brushed of airbrushed like Mig's paints. Tamiya XF are horrible brush paints though but great through an airbrush.
That a said though, got no idea how this all works on lexan.