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Also worth noting that using "the smallest aperture" can also be a problem. All lenses work best at an 'optimum' aperture - usually around f/8 - and using the narrowest apertures can produce lens diffraction and reduce the quality of the image (which already has its work cut out with a dark filter in front of it). Open up by a stop (at least) for better quality - say f/16 or even f/11. Yes, it will then make your exposures shorter, defeating the object to some extent..... but it's best to leave it to the filter to force the slow shutter.
Then, of course, a long exposure can also reduce the quality by introducing noise (becoming less of a problem with newer cameras) and the fact that a long exposure needs a very rigid camera - the longer it takes, the more chance of camera movement.
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