Wood – What wood should I use on a garage door

I agree, it sounds like a veneer that has aged over the years, dried up and is delaminating. One of the problems I see trying to repair the damaged areas with a filler, is that other areas may soon suffer the same problem. With that said, if you want to attempt a fix, you could remove all the loose material, sand off any remaining glue etc , then fill the voids. You could use a plastic wood filler, a thinset type of material or even a setting type drywall compound. Fix it muck like fixing a hole in drywall. spread the filler on with a wide knife, and sand it smooth after it dries. Be sure to prime and paint the entire surface before allowing it to come in contact with moisture or the outdoor elements. This may be a semi long term fix, but think you will continue to see problems in the future. Consider a new door soon.

To answer part 2 of your question.

You should probably reapply the stain a little earlier than the full 5 years - it could be starting to show signs of wear before then and you don't want to let the weather in.

If you are reapplying a stain then cleaning and lightly sanding the door should be preparation enough. If you want to apply a lighter stain then you will have to remove more of the old finish first.

Even if the stain does peel (which is unlikely) then sanding should be all you need to do. Be careful though to make sure that you sand the door evenly. If you take more off in one area than the rest it will show through.