Hi Bas - sorry, i forgot you're from the Netherlands and may not have heard the word "meths". To try and avoid any further confusion, here's a link to Wikipedia which actually calls it "denatured alcohol" but also gives other names for it and the general composition :
https://en.wikipedia.org/wiki/Denatured_alcohol
Up to about half a tank of petrol i would add about a litre for "serious cleaning" which is probably what you'll need if this is the problem. If your tank is full then add about 2L. If it's quite a way below half a tank, add a litre just before you add a few litres (5~10) of fresh petrol to help it mix in.
For a "routine clean", i add about 500ml to the tank immediately before adding 15~20L of petrol which i do once or twice a year, usually in autumn but sometimes in spring as well.
Usually i add a bit of fuel after adding the meths to help it mix in and make sure it's all in the tank then the fuel system does the rest. If this is your problem you should notice results fairly quickly.
The way it works is the meths forms an emulsion with the water and as the meths part will burn, it takes the water through with it. Don't ask me the chemistry, i scraped a pass in "O" level Chemistry too long ago to even try to remember, i just know it works!
