Flour seals, aka Distiller's biscuits, are use to seal dry fit joints or seams against leakage. Ethanol is highly flammable and you'll also lose a lot of product if you don't seal the still up tight. Mix 3 parts Flour to one part water. It should make a slightly dryish bread dough. If it's too wet add more flour, if it's too dry as a little more water. Roll it out into a 1/2" log. Once the still starts getting almost too hot to touch add the paste to any joints. If it's too wet it will be hard to remove from the still, if it's too dry it won't want to stick.