Dont add ground rubber, this weakens the epoxy. YOu dont want it weakend and it also will stop the boat from sliding over rocks and limbs.
I used 36 grit to grind the hull with, I also bucked all the rivets and then hand sanded around then, applied a bit of epoxy on each and every rivet.
Then applied Steel Flex to the whole bottom of the boat.
It is totally sealed and not a leak anywhere.
I fish White River in Arkansas and the river level can be low and draggin rocks now to 12 feet deep in an hour, and there are limbs and trees also in the water and I slide right over those like glass, no more like greased goos poop, I mean its awesome wish I had done this three years ago!!!!!!!!!!!!!!!!!
Be sure and mix the Steel Flex well, the more you stir it the better it will be, when you think its mixed well mix another ten minutes.
I used a small cheap brush to appliy it with and my boat is 22 feet long and 34 inches wide . ONLY APPLY ON THE OUTSIDE OF THE BOAT!!!! Someone in another post mentioned on the inside, hell man you will not be able to stand up in the boat.
JOhn