The ones that I made/sold only had black lines where you cut, not folded so that was never an issue. As soon as you make the cut the line disappears and the folds are free and clear of lines.
I’ve use resin ones when they came in an aftermarket set but they never looked as good as real cardboard to me.
Mine are also much darker in color and a much thicker, more rigid stock than those thin paper ones you get, and folding them up was no harder than making fruils or bending etch etc. but wutevs…
and yes as said prior, you can stain these with washes etc.