A follow-up with pictures. Here's what the black rubber 3M Scotch Mastic Tape (2228).
Amazon link. You can see where the rear wheel kicks dirt, rocks and sand onto the main triangle. I need to address the chain stays, as I ran out of the tape. But there is tape all the way up the seat tube.
Here's an image of the clear tape on my top tube. I run the tape running along the top tube, from the seat tube intersection to the head tube. You can see the edge of the tape below my cosmic horror (
Quasar )sticker, you can see a small bubble near the right side of the image, on the top of the top tube. When I installed the tape, there were hundreds of air bubbles; over time the bubbles faded...
You might be able to see some tape on the down tube, I put there for protection from tailgate pads...
I hope this helps thinking about DYI paint protection for their bikes.