I read a suggestion that one can glue coarse sandpaper to the inside of a small box like a matchbox, then close carefully and just shake hard for a while.

I remember a few years back a member of "that which will not be named", had actually tried gluing different grades of sandpaper to the inside of a rock tumbler and reported back poor results even after days of tumbling.

For super thick hardcoat large seeds like canna and lotus I've always gotten my best and quickest results with a dremel or hack saw.

Dog nail clippers work, too...

Karen

If you hold the sand paper and canna seed inside a large plastic bag----
the seed won't roll all over the room. Just sand until you see a white patch.
You can hold onto the seed with pliers.
Nail clippers also work to nick the seeds.

I pretty much use toenail clippers for everything else or cuticle scissors for smaller seeds.

I find patience works best and it is certainly the safest where fingers are concerned. I just plant the seeds, no scarifying necessary, water well and put the flat aside and forget about it. It is usually takes several months but the germination rate is exceptionally high and once the baby cannas have emerged, they grow quickly. I like to use the take out rotisserie chicken containers as mini greenhouses for starting seeds. The lid has perforations to allow air to circulate but they hold in the moisture well also.

I've had good luck with C. patens and C. warscewiczii this way.

Glue the sandpaper down. Now why didn't I think of that? I am going to make myself a little sanding block, with sides to keep the seeds from escaping.

In March, I wintersowed some old Canna seeds.
I did not nick or scratch them because I was in a hurry.
There are one or two sprouting now.
I was not expecting any to sprout since they were older seeds.

Caroline, canna is a hardcoat seed with a very thick coat and has a shelf life with an unbroken coat of at least ten years.

Sometimes I think that some older seeds have "used up their dormancy" and are more willing to germinate without stratification than newer seeds.

With canna seeds, maybe the hard coat becomes more permeable to water after a few years of storage.

Or it could just be that the seed coat breaks down over time? I started some very old seeds (although I DID sand them down a bit) and had very good germination rates. I swear, every seed is different!

Every seed IS different!

Plants are people, too.

My Canna indica 'Sunset' start germinating all over the place, without my help. They simply self-seed and grow in the spring. Heck, I have them growing in pots near the Canna, in the ground adjacent to the Canna, and any other place those seeds happen to fall on.

Hold seed in Vice Grips and use Dremel tool with grinding wheel. Works nearly all the time.

Some very hard seeds - soak in Stump removal solution for 2 - 4 hours and then do above.

If you still have a hard time, do all of above and use a small triangular shaped tool to create the indentation where you can use the Dremel tool.

Used the above methods on seed of African Dream Root seeds which ARE impervious and hard as Titanium and make canna seed seem like child's play.