TOM............... I replaced the headliner in TAMZIN, well what was left of it, about eight years ago with a plastic or pvc material I found at Homedepot.  The material I used was about an eighth of an inch thick and has sort of a bubbly or bumpy texture to it.  It came in 4x8 sheets and I glued it directly to the plywood overhead with teak strips to cover the joints.  The stuff is wonderful, was relatively easy to install and is a snap to maintain.  You've probably seen it in just about every gas station bathroom you've ever been in used as a wall covering.  I mention it because I recently noticed that they've come out with a board-and-bead finish that I wish I'd seen way back when  that might be a workable alternative for you.   

Thanks guys, I think I'll head over to Home Depot and check this material out. I like the idea of a pre-finished vinyl for all the reason you mentioned. I need to keep the thickness down to less than 3/8ths of an inch because my teak ceiling arches are 1/2" thick. The 1/4" thick Plastpro with 3" bead centers would work well. I'll keep you informed.

Anyone have any brand names for the vinyl/PVC panels people are discussing?  I'm not finding them on Home Depot.

Hello, i have installed it many times. You want to look for FRP. fiberglass resin panels. Or fiberglass reinforced panels. many accessories for it too. "H"channel to join two panels."J"channel or "cap".  And inside corner channel. I have installed it in walk-in showers with great results. The trick is to use  a bead of  silicone sealant or similar sealant under all trim pieces and in the slot on the trim piece if you want it to be water tight. you can also use the plastic rivets for large areas.  Hope this helps