I agree with Jim; I'd use a stripper like KleanStrip Premium to strip everything off the outside. Then sand lightly with 150 or 180 grit. If there are any dings or dents, leave them, don't try to sand them out if they are very deep. You need a product with really good UV protection; what you were using doesn't have it. Epifanes Marine Varnish is one product that does. It's a royal pain to apply all 7 coats, but it looks great and has very good durability. That said, it still requires a maintenance coat every year or two. I'm just finishing an exterior door made of Sapele, and I finished the outside with Sikkens/PPG Cetol Door and Window finish. It only requires 3 coats. More importantly, it is supposed to wear w/o peeling so that you can apply a maintenance coat every year or two w/o it building up into a really thick finish. I've never used the product before, but it is doing well on some houses on the same street I'm installing this door. Two of those have no storm door (nor would one likely offer any benefit and might even make things worse) and are directly exposed to the elements and face due West. WNY goes from 90F to 0F, with plenty of rain and snow, and even weeks of hot sun in the Summer despite what many folks in the country might think. Both products are easily applied with a brush, an important consideration for a homeowner when it comes to maintaining the finish.

John