Hopefully you didn't wait, and cleaned up the interior with mineral spirits...
Those paint stirrers that chuck into your electric drill work well, but if your filler has been sitting a while, it does take a long time to get the silica back in suspension. It settles out quickly, and packs at the bottom really dense.
Given your canvassing woes, followed by the filler woes, I'm wondering if you used a really lightweight canvas?
Not sure what the best remedy is. If there is a lot of weave left to fill, you might just try ordering in some more filler and give it a second go 'round. If there is only a little weave left to fill, you might let it cure and then finish smoothing either with a 50/50 mixture of lightweight spackle and latex house primer (what we use after filling with ceconite), or a high-build primer from one of the marine paint manufacturers like Interlux. I'll be curious to see what others suggest, there are probably lots of ways to fix this.... In any case be sure to report back here what you decide to do.
Best,
Dan