As has been said already. Lower the ISO to the lowest point, stop down as far as you can go within reason and load the lens with a stack of ND filters. You will need high quality ones to prevent image degradation. Also with all that glass on the front, keep it out of the sun or you will probably have flare and ghosting.
If you are trying to do cloud trails and swirls you might want to use a technique that I use for making long exposure star trails. Stop down your lens add some ND filters (in that case use a few ND filters to give you a few seconds) and then take tons of photos. Stack them in photoshop and if you use the right blending modes you will get a perfectly exposed image that will show the time lapse. In this case, each image will add a little to the previous. Works well for star trails and I have used this on water as well to blur the movement in bright conditions where I didn't enough ND's on hand. I am sure you could also do this with clouds.