I've always been skeptical of paint sealants. I've also found that Griot's Garage products are excellent. So When they introduced their Paint Sealant I decided to try it on our Mini.
http://www.griotsgarage.com/product/...C+16+ounces.do
This is the new version with polish.
Our Mini lives outside and although it has a Noah cover on it for storms with high winds (to protect it from flying twigs) and during snow, it is uncovered most of the time unless it's freshly washed. It's dark blue with a silver roof and here we have really bad acid rain.
I clayed and buffed the car (I use a Porter-Cable buffer) and then applied the Sealant a bit over a year ago. There is No water spotting or acid rain damage to the car anywhere. No sap or bug juice spots either. It can also be waxed over so water will bead up and make routine washings easier. The new one-step sealant adds a polish to give that benefit to save a step.
Be sure to clay before applying anything to your paint. You'll be amazed if you've never used paint clay.