Clench nailing is a little bit tricky to get the hang of. Most people make the mistake of driving the nail home and then just smacking over the tail on the inside. (Incidentally, this is exactly how all the historic wooden commercial fishing boats in BC are fastened, except they use heavy galvanized iron nails which you couldn’t roll over even if you tried.) Yes, this simple method will work but you will soon come to find out that getting that nail end to stay flat against the planking is a challenge. The tip will tend to protrude back out and no amount of hammering will fix it. Rolling that tip over at the onset is the critical detail and manipulating your bucking iron just so is the key.
I use an auto-body dolly as my bucking iron. Its combination of a well rounded face combined with a flattish face seems to be both comfortable to hold and effective.
You might be able to get a small lump hammer head or sledge to do the job but a good alternative might be a short section of mild steel round bar. About 4″ – 6″ long should do it. Crown one end a bit and dress the other a little flatter. Round over all the edges nice and smooth. While you are at it maybe add a 3/16″ diameter countersunk hole in one end for riveting.
Clench nails are sometime referred to as “clout nails” but I believe that is an incorrect name and you would be wise to see a picture of what you are ordering before committing. What we are looking for is a cut nail with a swollen portion and a flattened tip. You could get away with a round wire nail if it has a flattened tip, sometimes called “canoe nails”.
If you are using soft planking like red or white cedar you will need light gauge nails. The ones I use are measure 0.060 in cross section. About 14-16 Gauge I believe. I have tried using heavier ones and they are considerable harder to turn without a solid surrounding material.
Finding the right nails is probably the biggest challenge and I had a hard time tracking down any sources recently but I did find one, which happens to be the first boat supply place I ever walked into many years ago. Tendercraft Boats