RO (reverse osmosis) typically goes up-stream from a DI unit. The reason, an ro unit will drastically reduce contamination levels of any water by around 95 percent. Once the levels have been reduced, the DI resin will last so much longer. If your water is not very hard..say under 75 tds, then an ro unit wont ad much life to a DI unit so money would be wasted. Find out how hard your water is and proceed accordingly.

Now, the problem with pure water and windows is technique not water. Cut-ups or french windows require more work period...no matter how you clean them BUT with DI water, less detail work is required after cleaning. The reason one would go to DI water is for less ladder work, not less work. If you putz around with DI water and some different window situations you can truly get a feel for when to use and when not to. For example, try not to do window with Di water in the direct sun...water wont get a chance to sheet off and will spot. High wind days..also, be carfeul not to get the area above the glass very wet, it will carry contaminants from the edge onto the glass and casue spotting. again, it is NOT the water that spots...it is what the water has carried away from other areas on the glass that spots. Hope this helps.