The difficulty here is to allocate the distances between the lines and the line weights appropriately. If done incorrectly, the single lines black in for the viewer. This is especially important, when it comes to scaling the illustration.

It would be more appropriate to draw these lines with a thinner line weight than the rest of the illustration, or use a grey line instead of black to simulate a finer line impression. But be careful: if the color is too light or the line too thin, the part may begin to blend into the background.

Since you certainly don't want to draw each line separately - imagine how much time this could take! - you can define a certain area where a regular structure is inserted. Then, you can copy this section as often as you like and connect these areas. The brush in figure 3 is a good example. To achieve the "irregular" impression, you can later add a few single bristles to the brush manually.

It is also quite elegant to work with masks. This is an option offered by almost every graphics program. A mask is a closed path into which you can insert regular or irregular patterns or grids. The major benefit is that the structure is not adjusted to the mask, which means it can be much larger than the actual mask.

These examples may give you an idea of how to proceed in such cases. After experimenting with several methods, you will be able to determine what suits you best and supports the overall impression you intended with your illustration.