Originally Posted by
John TenEyck
Rich, if your door gets any significant amount of direct sun on it I would not put a storm door over it. That often is a recipe for significant damage, such as split raised panels, up to joint failure. If you do decide the best option is to add a storm door then I highly recommend you buy one that is vented so that the temps. between the doors is reduced. Also, I would put one of 3M's UV and heat blocking films on the glass to further manage the temp. gain. I had to take both of these actions on a door with a storm door that I couldn't convince the homeowner to remove after the panels cracked.
Now for the mold. Sanding probably won't eliminate all the mold, just what you can see. I think the only way to really kill the mold that's down in the pores is to use bleach or something similar. Whether or not the Spar varnish failed is hard to say, but if you couldn't remove the raised panels to kill the mold on them where they fit into the frame then you can't blame the varnish. I think the only way you are going to resolve the mold problem completely is to remove the panels and kill the mold on all surfaces of the panels themselves, where they fit into the frame, and the stops that hold them in the frame.
Once the mold is killed you can pick your poison as to what best to finish it with. I've had really good luck with PPG Cetol Door and Window finish, but there is no reason a Marine varnish shouldn't work well, too. I guess another option is no finish at all. The torrified wood won't rot, and having no finish on it would allow you to wash it with soapy water and bleach as often as needed to keep the mold off it.
John