Hi Michael,
The filler should be fine, but you will need to work this problem yourself.
Keep in mind one of the main ingredients is silica. It will be like a rock at the bottom/top of the can. No shaker will loosen this stuff up.
You get yourself one of those paint mixer rods that you put in an electric drill. Save time and use the drill with a cord because this may take a while. The mixer end need not be more that a few inches in diameter with flutes etc.
Open the can and pour off a few inches of the liquid into another clean can or container. You will now have room in the filler can to stir this stuff and not get it all over the shop.
You will notice the stuff at the bottom is very hard. You use the mixer rod like a drill and start in one spot and drill to the bottom of the can with the mixer rod. Draw it back up and "drill" another hole into the filler. You continue to do this until you've drilled most of it. Now you can use the mixer rod like usual and swill the stuff around. Then you can add the liquid you poured off and mix it all together.
Good luck,
Paul