Ben

The short answer:
You have to set your aperture up as high as it can go. For my cam, that's f22 at 18mm focal length. Make sure your ISO is all the way down as well.

A different answer:
First off...You're shooting in day time. It's not easy to get a rightly-exposed shot in broad daylight(Especially in bulb mode when most of the time people hold the shutter open for more than 30 seconds). I've found even at F22 with the shutter open for 10 to 15 seconds its still way over exposed.

So what I suggest doing is again, setting your aperture as high as it can go, then only exposing the shutter for 5 seconds at a time. If it's underexposed after only 5 seconds(or even less), increase the amount of seconds you're exposing the shutter until you get the desired amount light in your picture. After getting that desired amount, continue your shooting after each X[amount of] second[s] exposure. Don't waste any time between shutters.

And remember to put the ISO down as low as it can go--As well as any Active D-lighting...etc.

Let me know how it goes.

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