Although it sounds like some are getting a bit uperty, their 100% right. Many times one sees the same questions which, once you start actually building your outside train the answers are rather obvious.
Forget involving hours and hours of study, experiments, ans over planning, just do it.
If you want to see what you can do with absolutely no experience in garden railways, and no one to ask, have a look at my web site www.gscalechuffchuff.com
Its far from perfect but everytime I do something I learn something new, somethings work, some not, but at least do it.
Dont get offended, but its really overkill at times. In NZ we are the closest civilised country to the large hole in the ozone layer and we have the strongest UV rays in the world. But as far as models are concerned, whats the big deal. Nothing last forever and instead of putting plastic models etc out in the sun as an experiment observe the humble plastic ice cream container. Left untouched, about 5 years in the sun… so that is how long your models will probably last for . Perhaps 10 at the outside, so plan to replace them in that time.
Just like us mortals everything deteriates with time, so stop thinking how to protect it to last for the next 100 years and get out there and do it. If your young and have 50 or so years ahead of you playing trains by all means build it like a house but how long do you expect to be able to play trains.
Once you start you will find it far more fun and challenging that studing up all the time because every day someone will have a different point of view so unless you actually do something you will never know who is right. You can always undo what you have done and start again.
If your worried about how long the plastic will last for, wait till you start worrying about keeping the track clean. Lets face it, its outside, things like to walk on it, do other things on it, leave just love laying on it, and if you plan 3 rail o scale,