I have set up a documentclass like this
\documentclass[
11pt, % Font size
final,
cleardoublepage = plain, % just for page numbers on cleardoublepages
parskip=half, % Half a line skipped between paragraphs
twoside, % Two-sided document
openright, % Chapters start on right pages
bibliography=totoc, % Bibliography in ToC
listof=totoc, % LoF and LoT in ToC
footsepline=0.4pt,
]{scrreprt}[2015/09/15] % Use current version of KOMA-Script
% Author and title of document
% Header and footer
\usepackage[automark,headwidth=textwithmarginpar,footwidth=text,footsepline=0.4pt, headsepline=0.4pt:textwithmarginpar]{scrlayer-scrpage} % Custom header and footer
\usepackage{lastpage} %needed for page number
\pagestyle{scrheadings}
%\setfootsepline{.4pt} THAT DIDNT WORK__________________________________________
\renewcommand{\headfont}{\normalfont\sffamily}
\makeatletter
\renewcommand{\chaptermark}[1]{\markboth{\@chapapp~\thechapter~--~#1}{}}
\makeatother
\rohead{\rightmark}
\lehead{\leftmark}
The footsepline appears, but only on pages wich a) contains text and b) arent the first page of a new chapter. How do I change that, so that footseplines appear on every page with a footer?
\chapappinstead of\@chapappyou won't need\makeatletterand\makeatother. And\rohead{\rightmark},\lehead{\leftmark}AFAIK can be replaced by\ohead{\headmark}. BTW: You should always show a MWE instead of only a snippet from the preamble. – Schweinebacke Nov 06 '17 at 06:51