I have a '98 with a plastic bedliner. There's plenty of places on it for water to leak underneath. Can i expect to see a lot of rust underneath there?

 

There's plenty of galvanized steel under there. I would expect to see more paint rubbed off at all of it's contact points rather than rust.

 

and it happens fast to. got a plastic for sale by the way

 

I have the Ford branded liner and have had them in all my trucks since 1987 and have yet to experience any problems in any of the trucks like some are warning about.

When the liner was removed from the '87 for repair to the body and bed because of an accident, the bed was dusty but otherwise in like-new condition. Same with my '97 when I had the liner moved over to my '99. My '01 SuperCrew has the Ford liner and I'm not worried at all about it.

 

I have a Ford bedliner in my 2001, and have had them in other trucks through the years. I have never experienced any problems with them. I thought about getting a spray-on liner when I bought the truck, but there were a number of resons that I didn't;
1) they seem to always look dirty
2) you can't slide things on them
3) the texture isn't always uniform on them
4) if you throw something heavy-enough in the bed to dent it, you see the dent, not so with plastic
5) they can be gouged and scraped-off
6) once its on there...ITS ON THERE
7) if you keep the truck long enough for the bed floor to rust, at least it's covered if you use plastic
8) they're more expensive