I'm using \arrayrulewidth to specify my rule's width/thickness, e.g. in tables:
\taburulecolor{title}
\arrayrulewidth=1pt
\setlength{\arrayrulewidth}{1pt} % Definiert die Liniendicke von Trennlinien in den array- und tabular-Umgebungen.
%referenced by btable
\newenvironment{mycustomtable}
{
\begin{center}
\def\arraystretch{1} %Seems to be better than to use %\renewcommand{\baselinestretch}{1} % Zeilenabstand
\tabulinesep=1.2mm % cell padding according to http://mirror.ctan.org/macros/latex/contrib/tabu/tabu.pdf
\begin{longtabu}[l]
}
{
\end{longtabu}
%\renewcommand{\baselinestretch}{1.5}% Zeilenabstand
\end{center}
}
Changing \arrayrulewidth does effect the thickness. But: It's not fixed and changes from time to time. In some cases, \arrayrulewidth seems to adjust automatically within the same environment, e.g. after a pagebreak. Why is that and how can I specify arrayrulewidth to a fixed value that will not (never) change?

\documentclassto\end{document}in a form that shows two different rule widths. Then we will be able to say why they are different. – David Carlisle Jul 15 '14 at 11:38