Thank you for your comment, Ginny. I use a razorblade, but not a sand paper. Furthermore, I should have mentioned “density” or “sharpness” of lines when talking about the pastel pencils problem which has been bugging me. I want sharp and clear lines especially when I am working on something which has clear-cut edges like a house. I have just uploaded a new pastel work and for the very thin lines used for the café building, I used black or purple coloured pencils. I couldn’t help it, and as long as this sort of mixed media approach works fine, I lean toward keeping this way for my other works as well. At least I don’t find any reasons why I shouldn’t. Now I just did a bit of comparison of the thinnest lines I can achieve using a coloured pencil or a pastel pencil. This is the results of the comparison.
The line drawn using a coloured pencil is thinner plus more distinct, i.e. dense and sharp. This is the line I want. Maybe for the drawing of animals, the little bit of unevenness of pastel pencil lines would work as a strong point because the subject is soft and fluffy. But when it comes to buildings, and especially drawn using an A4 size paper, I think coloured pencils are the appropriate choice.