Inexpensive and so harmless you could eat it.. if you liked that sort of thing. You will need;
3 tbsp cold water
3 tbsp white flour
1 cup hot water
1 tbsp sugar
Put the kettle on. Mix up the flour and cold water into a smooth paste, adding some more water if needed. Measure out one cup of the hot water into a saucepot over medium heat, slowly pour the cold mixture into the hot water, stirring constantly. Bring to a boil until paste thickens, turn off heat. Add a tablespoon of sugar and mix well. Allow to cool, then cover and keep refrigerated until needed.
When using the glue, brushing a layer of paste over your poster will make it more weatherproof, but if durability is super important, you could add a little bit of pva glue (white glue) once the paste has cooled.
Recipe from Keri Smith's book, 'The Guerilla Art Kit'.
