Belled ends make a stronger joint than a fitting. White PVC is also sold with belled ends, but the gray conduit has UV-inhibitors. Consensus is to paint pvc pipe with white latex to avoid poly degradation.

Part of the reason I went with top rail is that the only expensive fitting I had to buy is the "purlin clamp" that goes at the top of the hoop. I paid $3.50 each. Top rail is about $1 per foot.

Thanks for the feedback. I'm actually thinking about building four foot high sidewalks from cedar two by fours that I can get very cheap and put a PVC top I get an eight foot clearance for a fifteen foot span

Hello veggievickie, you can try this site

http://www.snapclamp.com/prices.htm
or this one
http://shop.hoopbenders.net/connectorsandclamps.aspx

I've read about the supposed poly degradation too. My experience so far (three years using unpainted 1" white PVC) is different. I can see nothing bad happening at this point. Perhaps next year since the film is rated for 4 years :-)

In constructing an adjacent high tunnel I used the grey PVC conduit after recognizing its UV stabilizers. But I didn't paint that either.

I should add that I do not live in a climate with hot summers, so that may have something to do with my results. The poly film itself contains UV inhibitors so that may also account a little for my results thus far. I figure that if the poly lasts its claimed 4 years, then I have gotten my money's worth without the nuisance of painting the pipes.