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I'm working with an architect to design some coverings that will protect exposed beams and rafter tails from direct sunlight and weather. My inclination is to use metal, and I probably will. However, she asked if I could 3D print something for the job. I don't know of a plastic we can print with that would withstand that kind of punishment, but the idea is interesting, and I want to make sure I'm not missing anything.

I considered electroplating, but I think the daily expansion cycle would flake off the metal. I also considered using paint, although I'm not sure how well that would hold up either. Is there some process that can be used to harden a 3D printed part against weathering? Alternately, is there some exotic filament that can do the job by itself?

What about Taulman Nylon, perhaps with an ultraviolet protectant on it? Nylon does absorb moisture but that should't be a problem for this application. The other option is to print in ABS and then cover the print with some laminate, perhaps copper. Basically, use the 3D print as the form and put a shell of something formable over it.

could you print something then coat/vacuum seal with some resign? fiberglass is all weather so you could probably use that 2 part epoxy.

Fiberglass epoxy sounds like it could work. Metal covering would have to be quite thick, as the plastic will expand and cool thousands of times throughout the life of the finished pieces. Whatever I do would have to have some advantage over just using metal, and I already have a CNC router.

Maybe SeeMeCNC will come out with a selective laser sintering printer. That would solve a few problems.

depending on your item, you could print it without a top layer, then pour the epoxy inside too

Unless you've actually got a house-sized printer like these guys (http://3dprintcanalhouse.com/), your job sounds too big for most 3-d printers. For time reasons alone, even if you could print enough sections that fit together to make big parts. You could print spacers for metal flashing, since it's good to have some airspace under flashing to avoid trapping moisture.

Of the traditional solutions, metal is probably best, next to no maintenance. A couple coats of good resin-based paint is ok, but would need renewing in 3+ years in full weather. Covering with marine-grade plywood is also a fairly cheap solution...have the top overhang the sides with a groove underneath to create a drip rail. That may be good for a decade with no maintenance, or forever if you repaint regularly.

But don't let me stop you from trying to find the perfect plastic.

I've heard of polycarbonate used as a siding material. Imagine printing this.

I think metal is the way to go. Copper should perform well, and take on that pleasant "old-ish" green color as it oxidizes. I can use the 3D printer to fabricate some jigs for flaring up the drip rails, but that's probably as far as I'll bother.

Sprayable Plasti-Dip is amazing for weather protection. I use it to customize cars instead of painting them. Protects against the elements, and is completely removable. Not sure if it is right for your application, but I'd look into it.

It's really affordable, and you can use the aerosol cans from the hardware store, or the sprayable gallons with an HVAC sprayer.

Print something and then make a mold of it when you're happy with the result. Then use the mold to cast with architectural materials, like http://www.smooth-on.com/duoMatrix™-Sys ... index.html. If you're not doing anything elaborate with design and just going for utility, epoxy or metal is still the route to go though.