I think you can still buy "Deep root weedkiller gel" made by "Growing success" although the jar I have must be 2 years old.
IF you make your own mixtures, and IF you decide to store thsm in containers which are not the original ones, get heavy handed with the home-made labels! (and keep it locked up, and in every other possible way, out of reach of children).
If you wanted to mix wallpaper paste with Roundup, I would put the mixture in the Roundup container, and write something like "+ wallpaper paste" somewhere on the label.
The other possibility is a 'weed-wiper' which you may be able to find in an agricultural shop. You mix the roundup much stronger than for spraying, and fill the 'handle' (which is the reservoir). Useful if you have a LOT of weeds, but don't want to spray. I used one to get rid of a patch of ragwort from a paddock. It is especially helpful if the weeds are the tallest plants in the patch....

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Flowers come in too many colours to see the world in black-and-white.