Friend of mine went with silicone the panels and the extra clips and screws,and said she was glad she did! Not only that but braced the corners with galvanized angles,helped with keeping the doors in check and working properly.

Thank you dbrya1,

I never gave any thought to reinforcing the corners. Any idea how long your friend has had that greenhouse? and how are panels holding out. Any cracks or other signs of deterioration?

I think 4 yrs now,2 yrs while living in Ks and 2 yrs now since I moved to Texas,but they put it up first yr,just like normal,then the winter winds blew out couple panels,and tweeted the frame,making it hard to open and shut the sliding doors,so come spring they went into action and made those changes,been doing great every since then.She also ran channel frame the length and width for hanging plants,but I bet it also added to the strength of the greenhouse.Make sure you anchor to the ground,or it will go up up and away!

Hi oldlady59, our 10x12 has been up 8 years now, in a windy climate, using the screws and extra clips method. I didn't want to use caulk to secure the panels, because I like the flexibility of being able to remove them (the south wall of mine is replaced by screens in the summer time.) But I've heard good reports about using the caulk method, too. I don't know how hard it is to remove caulked panels, years later.

People seem to report different experiences with the panels; my only guess is, it's based on climate and how severe the sun exposure is. Maybe there's even some variability in the manufacturing of the panels? I dunno.

We had to replace our roof panels after a bit over three years. They were under Aluminet shade cloth, but our very intense sun in southern NM took a toll, and they were yellowed and full of small holes in the outer layer. Most of my side wall panels are now in the same state, but it took those almost 8 years. The panels on the north side of my greenhouse (sheltered by our house and interior insulation) still look like new.

At the end of this thread, there's a discussion about a clear product that may be helpful in extending the life of the HF poly panels. I've used it and so has another GardenWeb member. So far it does seem to help, although I can't say for how long (time will tell I guess.)
Harbor Freight Greenhouse polycarbonate panels failing
When I order new side panels, I'll be using this product on them before installation.