The best "rust shield" is a can of paste wax and elbow grease. Apply twice a year, Spring and Fall, and you will never have a rusty tailgate. Just raise the tailgate to eye level and wax all of the metal you can see.
It infuriates me to no end when I see perfectly good 245s with all of the paint rusted away because the owner fails to maintain his/her car. The cause of the rust is not water, it is neglect! Same for the bottom of the doors. And don't expect a "detailing" place to wax any areas of the car that are not readily viewed.
Covering the bottom of the tailgate with a strip of plastic will only trap water between it and the paint. It will rust even faster next time!

Klaus
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(V♂LV♂s 1995 854T, 1998 V70R)