Sorry, couldn't resist 
One nail per lath and board when it comes to tongue and groove paneling... the tongue holds in one side and the nail is placed 2/5ths in on the other side... if you use two nails, the board will crack much easier when the paneling tries to move! You can never try to hold the paneling fixed, because it will move anyway, and if it's attached the "wrong" way, then it will crack!
Nowadays, the timber is of quite poor quality. In the past, logs were carefully selected before being split into various dimensions. The timber was turned so the core side faced outward. This no longer exists nowadays as everything is automated. When I was a teenager working at a sawmill, we sorted out poor quality timber into a bundle called utskott. These were boards with many large knots, etc. Nowadays, you have to set these aside yourself when purchasing timber at the lumberyard. It's difficult to find boards without unsightly knots or other issues that cause the boards to warp and twist as they dry. Often, I try to sort out the utskott when choosing boards, but many people take them home and are surprised when the boards become like propellers after a few weeks. Therefore, you should place two nails above each joist along the board and set them about two cm in along the tongue or groove to prevent the board from cracking at the edge. One nail in the middle would allow the board to warp and make the wall look like flat gutters attached to the wall, and cause the nail to creep out. Showerhill makes a clear point with what he writes, but I would only use it on dimensions 21*70 or max 21*95; otherwise, I would use two nails to ensure the wall stays intact.
Yes, then one might be tempted to place 3 or even 4 ...
Yes, a nail gun is definitely one of the most time-saving tools you can use. The advantage is that you can attach quite a few boards with a few hand-driven nails. Just make sure to knock the boards together properly while ensuring they are aligned vertically. When you then use the nail gun and follow up with additional nailing, it's fun to work. Be careful not to crack the boards; if you have too high air pressure for the gun, there's a risk it will drive the nails in too far, making painting difficult and damaging the wood. I usually have the pressure a bit too low so I’d rather need to hammer a few nails than have them go in too deep. I assume you're using 75mm nails, preferably grooved.
My nail gun allows you to adjust a small "mothåll" by the nose to set the nail depth. It works, even though it's some cheap version of a gun.
I still adjust the pressure as well, to just barely get the nails in at the right depth, not sure if it's unnecessary to have both belt and braces.
So there's still some post-nailing work.
But I'd rather have that than a lot of deep holes in the façade everywhere.
I still adjust the pressure as well, to just barely get the nails in at the right depth, not sure if it's unnecessary to have both belt and braces.
So there's still some post-nailing work.
But I'd rather have that than a lot of deep holes in the façade everywhere.