It is possible to make your own devitrification solution. It is made from borax which you can buy from your local chemicals supplier, or sometimes as a washing powder – but make sure it has no additives! To make a solution, boil a few cups of water. Take the water off the boil and put in 4 – 5 tablespoons of borax. Stir and allow to stand until cool. Pour off the clear liquid and you have a saturated solution of borax. The sediment in the bottom can be added to more hot water to make more of the borax solution.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.
You spray this solution onto the glass, just as the commercial sprays. It requires an even application to ensure there are no streaks left in the finished glass. Borax is one of the fluxes used in glass making and serves to soften the surface of the glass enough to overcome mild devitrification.