Maybe you could put polyurethane on them. I've never used the poly, but I don't know why it shouldn't work.

I have a dehydrator and I cut my apples in slices and soak them in lemon juice, I have even saw where they soaked them in salt water. Then pat off with a paper towel, and put them in the dehydrator. Some people dry theirs in the oven. i've never tried that.
Here's a link that might help.

Also, here's another sight.
http://www.ehow.com/how_3906_dry-apples.html

Here is a link that might be useful: drying apples

I did them.I have an Aga stove so I sliced them thin then put in low oven for few days.When done I put Poly on them.They lasted a long time.

Thanks - I'll try putting poly on them when they've dried.