Delve into the foundations of Country Guitar and equip yourself with the skills to play this genre with confidence! Full lesson here!
Hi!
I love the sound of this, though I find it difficult to use the index finger on the upstroke. Although I keep practicing it still doesn’t come naturally. I find that I inadvertently use the thumb on the upstroke: this comes more instinctively, seems to fit with the strumming motion of the arm/hand and seems to sound good. Is this acceptable, or am I just storing up problems for later?
Hi Martin @bitofaterrier ,
Welcome, and I hope you have a lot of fun… 
I would definitely recommend learning it the way Justin shows here right from the start; if you enjoy this style (I love it), you’ll really need that index finger later on to pick out notes (and melodic lines) in between—plus, it teaches you a better technique for keeping your hand in the right position…also Later in the course or module, you will likely quickly notice that if you use your thumb for this, the palm of your hand cannot properly ‘mute’ the strings when necessary.
I promise you that if you give this a fair chance—even just briefly every day (or at least a few times a week)—you’ll very quickly forget that you ever found it difficult… 
At the 3:24 timestamp, Justin states it very clearly… 
Greetings,Rogier
Hi Martin and welcome to our Community!
Rogier gave already very good advice, all I can add is that it is pretty normal I think to find the upstroke with index difficult because you need to train it to do a movement it has never done before; just keep practicing and it will gradually become more and more natural.
Experimenting and exploring sounds to find out what works and sounds better to your ear is exactly the point of strumming using fingers and I would encourage to keep doing it - though this specific technique is very worth to be learned the way Justin presents it - once nailed you’ll find yourself able to further explore and add your own variations.