I'm using this and this approaches to get my appendix sections as I want, so a MWE of my code would look like:
\documentclass{article}
\pagestyle{headings}
\begin{document}
\tableofcontents
\clearpage
\section{abc}
def
\section{ghi}
jkl
\appendix
\clearpage
\section*{Ap: ETC}
\addcontentsline{toc}{section}{{Ap: ETC}}
\renewcommand{\thesubsection}{A.\arabic{subsection}}
App. content.
\subsection{First A.}
F. A. content.
\subsection{Second A.}
S. A. content.
\end{document}
The problem is that this makes the appendices pages heading to be the previous non-appendix section's one. One option may be to use \pagestyle{myheadings}
and \markboth
but, is there any more automatic solution?