Don't know if it's needed to have a stronger dilution for hard water spots than the dilution for it when used as an iron remover and a cleaner on boats gelcoat hulls waterline and down. The recommended dilution for this is 1:32 with oxalic acid powder and water. Apply it by a soaked foam sponge of the solution or spray it on with a spray bottle or pump spray bottle. And let it dwell for 30 minutes just wet it a little more with a spray bottle if it's drying on you. Maybe there are different concentration of oxalic acid in the powder when you buy it or a stronger solution gets more fast working to desolve water spots. And what I know of it's only necessary to rinse it off very thoroughly as the water used when rinse dilute it down a lot so it won't be any problems. It's important to work in the shade or better in a garage. The warmer temperature you have the faster it will react so test to see when it's desolved enough water spots. And if not enough apply again. Be very carefull to get it on to the glass and if you have worn plastic trim or bare plastic trim. As it could etch and haze these parts. That's why it's easier to have a controlled application by a foam sponge or mf media. And to work in a garage so you minimize the overspray.
