You could use HO products outdoors, but it takes a lot of work to make it semi-permanent and mostly reliable. You don't have to worry about the metal rails, but you do have to worry about what will accumulate on them every hour of every day. Instead, the plastic ties are going to need protection from the Sun and from acidic or other solvent-based materials that can basically rot them over time. Ballasting with a water-proof adhesive will help, and painting the ties before ballasting will help. You'd have to solder feeders every two sections of flextrack, and you would need the power bus mentioned above.
You would want a protected enclosure for your terminal posts so that you can reliably hook up the bus to your base station or transformer.
I can foresee major problems with turnouts over time, and not long. Turnouts cause the most headaches indoors, let alone outdoors, so double the probability of mechanical defects if you use them outdoors. Even handlaid turnouts using PCB ties will have the copper cladding corroding in wet weather....you'll have green ties ere long. If you can coat the ties with an epoxy and then paint them, you might be able to keep them in good condition for several years.
But your biggest headache, by far, will be in the amount of track cleaning you'll need to do, not the least of which will be ridding the area between the rails of any number of problems, including what gets in the way of the flanges. Then there's all that rail surface cleaning so that your trains don't grind to a sudden halt about 12 feet from their set-out points.
Really, the best system outdoors would be a larger unit that can carry a two or three 6 volt batteries around with them for power. They should be radio controlled, not controlled through a bus. Larger heavier units in the range of 10 kg won't be affected by detritus on the rails and between them to the extent our 0.8 kg HO models would.
-Crandell
