Any strap-on rack can damage the paint due to road grime getting between its pads and the mounting surface and abrading the paint. Applying clear protective plastic film to the areas where the rack rests will prevent that. The same holds true for the finish of the bike where it rests on the rack.

There are many obvious pros and cons between using a strap-on mount, roof rack, hitch rack, etc. My preference is to install a hitch and use hitch mount racks. A hitch costs ~$150 and installation is an easy DIY project. Hitch racks that support the bike by the wheels/tires rather than the frame can be found for around $140 and up.

Check out the Curt hitch website for hitches and also specific installation instructions and videos to get an idea of what's involved. The following site has lots of hitches, accessories, bike racks of all sorts etc. http://www.etrailer.com/dept-pg-Hitc...form_Rack.aspx