Here is a fluffy Siberian long furred Silver tabby cat that I felted using #36 and #38 star felting needles.
Question: I recently received this question. When do you know it is time to go to the next size needle?
That is a really good question! If you have felted something to the point that the needle doesn’t do anything anymore, then a finer needle is needed. The finer needles are the higher numbers. A #38 star needle is good as it has an extra row of barbs which grab the wool better.
Question: What is the best size felting needle to use?
I think that there are no hard rules on this. My favorite is a #36 needle. Years ago when I started needle felting all that was easily available were #36 coarse needles. So that is what I used for two years and everything was fine. I always use a #36 needle. I use coarse wool for the core as it felts fast and provides a firm base to felt fur into. The coarse #36 needle is great for this. I also use a #38 medium star needle. This needle usually costs a little more but it is really worth it. I use this needle for face detailing as it is a shorter needle which gives more control for the little area of the face.
Fine needles #40 or #42
Once in a while I do use fine needles if I am using fine wool for a very smooth surface layer like skin. The thinner needles make smaller holes. If you use a #36 coarse needle the holes would be much larger on a fine wool like merino.
So all in all I really do believe that felting needle size is a personal choice and depends on the fiber that you use, how firm that you want to felt it and what you like best.