I agree with Rigman123 that the main cause is the bolts not being sealed up. I believe the main ones of concern are the handles that bolt through the deck material. If you notice the inside has a torx head type of bolt that is actually hollow. The end of the bolt is supposed to plug the hole up. Mine swelled up just below the handle and when I took the handle off it had rust around the outside of it. The bolts going through the aluminum frame are a little better off as any water runs down the inside of the channel and out the bottom when it is closed. I noticed how all of this works when I drilled the holes to reinstall the brackets for the railing on my new door. I too am having rhino liner sprayed on both my patio and ramp doors covering all of the torx nuts. As a bonus to fix how hot the black deck can get I chose to use a light gray color. The cost is about $300/door. Well worth it if it does the job.
Please update after you get your doors sprayed with some pictures and any lessons learned. We just bought a used '17 4200 and plan to use it regularly and this sounds like a must do if no issues.