Before you use a clay bar you\'ll need to lift as much of the contaminate out of the paint as possible otherwise you\'ll just drag it out the paint and cause some major surface damage.
Best thing, when it\'s dry hit it with some snowfoam, don\'t jet wash it off because you\'ll push any contaminates deeper into the paint. Spray it with several coats of snowfoam and that should lift a lot of it away and soften any thing deeper. Rinse it off, don\'t pressure wash it off then while it\'s wet use a clay bar, ideally a softer one as possible as the harder ones like meguairs will grab any contaminates and drag them across the paint. Might even be worth using some tar remover or degreaser to help.
It seems like a lot of effort but if it is as bad as you say and it turns out to be grinding dust or something like that, then the less time you spend with the clay the less time you\'ll spend ending up polishing and restoring the finish.