Have you tried greenhouse plastic with UV protection? The plastics you have been buying may not have that and degrade quickly in sunlight. The reason they seem to fail in the location of the PVC may not be from a chemical interaction, but because the PVC may absorb and give off heat that hastens the breakdown.

Here is a link that might be useful: 4 year GH plastic

The chlorine in the pvc destroys the poly.
Use hotspot tape or a couple of thicknesses of poly to act as a barrier. Alternatively,paint the pvc with a water base acrylic.

Here`s the technical details,
Polythene is essentially a hydrocarbon chemical compound and frequent applications of similar chemicals containing the same elements, which many agrochemicals do, will be absorbed by the film and will act as a solvent - de-activating the UV stabiliser and causing the film to degrade before its design life. The two specific chemicals that have this affect are sulphur and chlorine. Do not use chemicals that include these elements. If this is not possible wash the polythene with clean water after contact with any chemical.

I have 3 hoop structure up with PVC and greenhouse grade plastic. They are doing fine. Was the pipe new? I ask because, mine has been sitting outside for at least a year and has "out-gassed" the bad stuff before I put it up. I was told this was good way or paint the stuff.

Jay

jrslick...I build this greenhouse several years ago.... Bought the pvc pipe from a dealer ..it is the grey pvc.... i think that what the plans originally called for.I first used wal-mart poly... it lasted abt a year or so... started ripping where it came in contact with the pvc ....have used commerical twice did the same thing...I think i'm going to do what hex2006 suggested is to paint the pvc with water base acrilic paint and see what that does..Even used 60% shade cloth and that didn't help either. Jake

White is the best color, anything dark will heat up in the sun weakening the poly. The best option is to use a polyester based foam backed anti-hotspot tape which will fix both issues.

Hi, any updates on this?

You're right. The PVC pipe will react with the poly. A good solution is to cover your pipe with white felt. See this link. This has an adhesive back on it so you can stick it to your pipe. Then use ABS snap clamps to secure the poly to the pipe so it doesn't shift around. Good luck!

Good morning. Our green house is built from soft steel. We are replacing the plastic this year and want to paint the metal. Years ago we were told not to use rust paint as it would damage the poly. We used an acrylic paint but have rust starting. Any idea what type of paint we can use?

Use the rust killer paint. It probably won't do harm to the plastic, but the rust will harm the steel if the rusting isn't addressed. If you are concerned about it, behave as if the rust killer is the primer, and use a different paint over it that you feel is safer for your plastic.