I would only recommend using C-curl on curlier lashes which means you are getting the same bond as using J-curl on straighter lashes.
If you use a stronger curl on straight lashes you wont be able to get a good bond and the lashes wont look natural.....semi permanent eye lash extensions are what they are....they are meant to look natural, ie you can only work with what you have got...some clients have thick lashes and you can use the thicker extensions, some have fine lashes and you simply cant put heavy extensions on as they would be just too much....and the same with curls...you cant put a strong curl on straight lashes and vice versa because you can t create a good and long lasting bond.
We can enhance the look but you cant change what is there!