I'm finally able to start repairing and finishing the work my ROTSG (rotten, theiving sack of garbage) contractor left me with on my kitchen renovation and face a dilemma. Part of the kitchen has oak beadboard on the walls, but I know there is only one coat of polyurethane on it and I don't know if it's water based or oil based.

What I do know is that whatever is on there isn't much. There is very clear demarcation point where the stain is much darker on one side and much lighter on the other (apparently the stain was applied after the beadboard was installed on the lighter side and lying flat before being installed on the darker side.) This weekend, figuring I had nothing to lose, I went over the lighter part with the stain (horrible Minwax stuff) thinking it might "take" because the boards really don't feel like they've got much of anykind of sealer/poly on them. Sure enough, the stain took and dried just fine. It's a much better match now.

So question... I'd really rather an oil based polyurethane on the walls. Can I safely do that on top of whatever does happen to be there? Or do I need to find a way to figure out if the minimal that's there is water or oil and match it?

If the latter...how can I tell which it is?