Hi,
I have 2 Polar HRM's, and have had the same kind of issues. Strange readings, or very low calorie burn when I know I was really working hard.
I tried the creams, but did not notice any difference and with the fabric type strap, it can be messy, I do wash mine after each use, ( hand wash with soap), Maybe will try putting it on a little before starting out, I usually put it on and jump right into the workout.
I have the F4 and the F6, did not know about a setting that gives you percentage instead of heart rate, have to look into that. Would work really well with Cardio Coach workouts.