Full Wipe How-To needed

Can you maybe elaborate on this a bit more? I would maybe need some really step by step explanation. Is the factory reset, the one I can acces in the settings? How do i fill the internal storage?

A guide?, seriously?, @Tuep, you have virtually answered your own question. The prompts during the operation are very self explanatory, so, yes, the factory reset option found in Settings is the way to go!

Yeah seriously, because apparently just a simple factory reset is not enough. I found an answer from chemist in a different question where he suggested to use dd to create a big file with zeros. So this is what I did then. But again with lot of problems in between. Eg. after the factory reset it the jolla was set to an really old release and installation of terminal was not possible, neither updating to a newer release. A second reset did the trick. Then I managed to get the terminal and acces to dd, filled everything cleared it, and did two factory resets in hope the next person owning the device will not struggle with it. But this is somehow typical for Jolla and SailfishOS. Vague hints how something is done is often not enough for everybody. Therefore I never managed to get eduroam working nor vpn. But hey, I dont care anymore, I am of the boat sailors.

"Apparently a simple reset is not enough" ..?, care to quantify that comment?, who says it is not good enough?, or why do you think it isn't good enough?

Sure. First you can already guess by the answer of chemist, that apparently it is not enough. But for more details, see this: https://together.jolla.com/question/94095/reset-device-does-not-really-clear-device-memory/ The Jolla guys seem to use the same vague language as many and the information that all data is deleted means in principle, that you will not have direct access anymore to it. But it is easely restored. And thats something I really want to avoid when selling the device. Maybe for the records, the hint with dd I found here: https://together.jolla.com/question/136344/securely-erase-device-in-order-to-sell-it/

The fact that Jolla developers seem not to really care about such privacy things fit perfectly into my experience so far. We are unlike seems not to focus and reefer to privacy and power to the user but more to super hipsterism. Otherwise I can't imagine how they focus so much on UI accessorise etc. etc. instead of implementing PGP, VPN etc. etc.