need to cut the entire top edge off of a bedliner, its the part that sits on top of the bedrail. Any idea`s? Its about 1/4" thick hard plastic about 20Ft all around

Sawzall, jigsaw or air saw. Might even get away with a circular saw until you get to the corners.

Thin blade grinder

heated knife, like a tire siping tool.

I've used a Roto-Zip with a sheet rock bit for poly as well as Lexan with great results. Kind of a cut/melt process but I ended up with a smooth finished edge.

Air saw, nothing better or faster!

I’ve cut a ton of pvc floor boxes, flush to concrete or tile, or wood, using a vibratory cutter. Nice neat finished product.
Use a long straight edge to guide your cut.

Thanks

Common to cut vinyl siding with a fine tooth circular saw blade installed backwards. Maybe that would work and yield a straight cut?

Chainsaw done

Torch better than chainsaw.