Does the surface feel smooth after claying? Can you feel the dirt or tar when you feel the surface with your hand? If it is smooth, then the dirt and tar are most probably embedded into the paint and you're just shearing off the top with the clay and not really removing them. If you can still feel them, you can try using a stronger or more aggressive clay. Claying is mechanical cleaning, maybe what you'd need is to try chemical cleaning instead. For the tar you can try something that would dissolve them, like a tar remover. For the dirt, you'd need something that would emulsify and dislodge them, like a strong all purpose cleaner or a degreaser. But be aware that when you're working with chemicals on the clear coat, you'd need to make sure that the paint stays wet or else they *could* stain the paint. But nothing that a light polish or AIO (All In One) couldn't remove.
Regards.
