If you want the wood grain to be visible, then it has to be plywood; you can, for example, glaze it so that the grain remains, or stain it as a darker type of wood to achieve an "old-school" style - somewhat like an English manor.
Is the house old or fairly modern?
Is the house old or fairly modern?
No, I don't want the wood grain to be visible, but completely smooth painted with lacquer paintL Lärospånet said:
kind of like this one:
http://norrabyggodesign.se/wp-content/uploads/2014/02/bröstpanel-31.jpg
In that case, masonite is cheap and easy to work with and builds up less than plasterboard and chipboard. It's smooth too. Plasterboard also easily gets ugly marks, but masonite withstands quite a bit. Well tested for your purpose!
At Bauhaus, they have pre-lacquered masonite at 30 mm, so you don't need to limit yourself to just 6mm if it becomes difficult with the chair rail. https://www.bauhaus.se/vit-lackboard-101-matt-3-0mm.html
However, I think I've found a bröstlist that fits
https://www.byggmax.se/virke/lister/övriga-lister/bröstlist-bomull-vit-p10219
assuming that the bröstlist is for their panel boards that are 6mm
https://www.byggmax.se/virke/lister/övriga-lister/bröstlist-bomull-vit-p10219
assuming that the bröstlist is for their panel boards that are 6mm