Howdy Ivan, a 10
stop ND filter doesn't get you very long exposures during the brightest part of the day unfortunately, as you have already discovered.
There are apps you can get for your smartphone which will work out the times you need, the one I use is called Slower
Shutter, for iPhone, but if you are an Android user there would have to be a similar app. You enter the
shutter peed for a normal
exposure and then how many
stops of ND filtration you are applying. It goes up to 16
stops of application. Some, if not most all ND calculators only go up to 10
stops of ND filtration being applied.
There is a good table that can be printed off with
exposure times already worked out for you
here that you can download for free. It can be folded up and carried in your camera bag with you for easy reference.
A really good read that I recommend time and time again, whenever anyone asks about long exposures is
Joel Tjintjelaar's blog piece. I have linked to the section of it about gear, because just below where you will land on that page there is an image of how he tapes the top and bottom of his square ND filters, to avoid
light leakage there. In that same image you will also notice that he tapes up the side port on the camera body where you plug in the
shutter release, if you use one of those that needs to be plugged in because that is also a
light leakage point in some cameras. Definitely remove your eyepiece padding and put the rubber blocker in its place, it goes with the
Canon label to the inside of the camera and simply slides on from the top of the eyepiece frame. I keep mine in the same case as my 16
stop ND filter, so I know exactly where it is at all times when it's not in use.
AM was correct in saying that by using a smaller
aperture, like f/22, it will show up all manner of chromatic aberration and also every dust spot that's on your sensor. I know it seems to be the logical thing to do so as to get the longest
shutter time possible, but it always comes at a cost to your image quality.
If you have a circular polariser, you could always stack that on top of your ND filter and see how long you can get the
shutter to go through experimenting with times, but be warned it will create strong colour casts once you turn it beyond a certain point!
On my
Canon 6D I can focus through a 10
stop ND filter using live view and setting the
ISO to say 800 or 1600 and the
shutter to 30 seconds. If you do this remember to set things back to what you need for the actual shot though as you could easily burn your sensor otherwise.
I usually focus first, without the ND filter in place, then I apply a very thick and tight rubber band to the focus ring, overlapping onto the
lens body, so the focus ring can't turn by accident when I screw on or place the filter in place in its holder.
Hope that's of some help to you.