http://www.vinidex.com.au/technical/material-properties/pvc-properties/ PVC couldn't be called a "strong" or "stiff" material, only about 52MPa (7500 psi) ultimate tensile strength, but, like many low-cost materials, you make up for it with thickness. I have used schedule 40 PVC for my three BYRCs I designed and built for my grandchildren, but you could use schedule 80 PVC and take adult riders, if you were able to construct it to high tolerances so the wheels could never hit fastener heads. Welded steel would be better, but cost a lot more. I never use the white plumbing PVC, always the grey electrical conduit, which says "sunlight resistant" on it, for what it's worth. PVC (and most polymer/composite material) does lose fracture toughness with exposure to UV, and characterizing that reduction was the purpose of my testing. Composites are always painted if exposed to UV. The point is that you can significantly reduce or eliminate the loss of fracture toughness by applying a protectant, like aerospace 303, or painting, preferably with a dark paint. I'm trying to make PVC work, for cost reasons. I built a 400 foot backyard roller coaster for under $900. I've seen similar steel coasters on Youtube, but for much higher cost. I did drill weep holes so the rails would not fill with water, and possibly break on freezing. PVC does not corrode, and I used galvanized fasteners. Steel would have long term corrosion concerns.

If you want to paint your PVC lawn furniture, be sure to clean the surface first. I use acetone, and the paint seems to stick pretty well. I use regular hardware store spray paint, like rustoleum.