Facia

Formica from a countertop shop that is cut to a standard width bends for smooth corners and is stiff enough to hold a small toggle switch to push button.  It is very durable and requires little to no more  preparation.  It can be glued or screwed to the benchwork.

I had masonite backdrops and

I had masonite backdrops and facias on my old railroad for about 15 to 20 years with no cracking problems. I used drywall mud and paper tape with three or four coats of mud just like you would use on drywalled walls. I used a 12" drywall knife for the last coats of mud so the joints were about 20" wide and then sanded smooth. Primed and painted they blended in perfectly without showing.

In all fairness I have to tell you that before I retired I was a building and remodeling contractor and have a lot of experience doing drywall. While I had no problem with my railroad I have seen a few failures and cracks with drywall in houses. On the joints that failed almost everyone was where the drywaller used fabric or mesh tape. If a joint shifted the little squares in the mesh became diamonds and the mud popped and lifted right out forming a very visible crack.

In each case when I did my repair I cleaned out the joint and then applied paper tape instead and was never called back, so I have to assume that the joint never cracked again. Now, I do have to admit that obviously any shifting and settling of a new building will almost always happen within the first year and maybe paper tape if used when new would have failed also but I rarely saw it over in my forty years in the business, anyway I never was thrilled by mesh tape even though it was easier to use.

So drywall mud and tape worked perfectly on my railroad. Good luck and happy model railroading.

Sam

I just realized

I didn't answer all of your questions. On my walls I used drywall and in the corners I had mountains that went all of the way to hide the corners so no corner curves. In the center of the room I had a long serpentine peninsula that wound around for about 20' of the room where I had the Masonite and that was was screwed to 24" O.C. 1x4s so that it could flex at the top but with the curvature it really was quite solid. On the bottom edge after I fastened the cardboard strips for the scenery and then applied plaster impregnated paper towels they couldn't move. The Masonite joints were butted, glued and countersunk screwed to the 1x4s.

I live in Michigan and didn't have air at that time. There were temp swings and humidity changes, dry with the furnace on in the winter, humid in the summer, though I did use a dehumidifier in the summer and that helped. In a heated house in Michigan the basement will only average about 20 degree swings in temp with the coldest being maybe 60. F in the spring when the furnace is off but while the ground is still cold. Summer is the warmest but again with the cool soil conditions It might approach 80. F but only on the hottest days.

In your practices section you mentioned painting and mudding and then sanding again after the primer paint. I wouldn't, apply your mud and sand it smooth right on the Masonite (That's good quality Masonite and there is some Chinese crap out there that might need more help) To get your joints smooth using a bright spotlight held at an angle will help show you any imperfections, then prime and then paint.

The purpose of primer over drywall mud is that it's too "Hot" for the paint to stick and regular latex paint will peel. All you really need over the mud is a cheap PVA primer to tame the alkali, but be warned PVAs are cheap but don't hide well. If you want to use a better hiding primer paint as long as it says it is a primer it will work, then top coat.

Even though you can touch it and it feels dry latex paint takes a long time to fully cure and sanding latex wall paint that hasn't been on for months in my experience will make a mess.

I think that now answers all of your questions, Sam 

Taped joints

The backdrop on my current layout is masonite, about 1/8 thick.  I air nailed it to the studs with a brad nailer and stapled it along the top and bottom. The joints were butted together with a slight gap and brad nailed. I taped and smoothed the seams then painted it. After a few months a couple of seams showed hairline cracks so I scraped off the joint compound and the tape and sanded the face of the masonite where the seam would be then re-taped and smoothed the joint. It appears that the factory surface of the masonite was too hard and glossy for the tape and joint compound to get a good bond. The re-done joints seem stable so far. A couple more of the original  joints have cracked over the years but I haven't bothered re-taping them  as I rarely notice them anymore. So I'd suggest sanding the face of the masonite  well before taping and look for a joint compound that has good adhesion.....DaveB

Backdrop

here's some quick photos of a backdrop that uses the "best practices" in your opening post. It's been in place for 20+ years and the one before it in a previous garage for 10+ years ( California with temps and humidity controlled only when I'm out there). Neither ever cracked. The only differences from your list were the sanding of the joints was pretty aggressive and the priming was put on after all the sanding was finished.

I had to get a tape measure out to find the seam. The red arrow shows where the sheets meet.

Close up of the seam area.

Masonite backdrops

The equivalent of Masonite is no longer made here and imported. Seems to be OK though. Since I have a double-sided backdrop along a peninsula as well as backdrops along brick walls, my process was a little different.

For the double-sided backdrop, I used 25mm square (1 inch square) aluminium tubing from my local hardware store. It is used for shelving and similar projects and I checked that Home Depot also stock it. It is cheap, easy to cut and drill. I assembled it into a frame which was bolted to a supporting L-girder beneath my plywood benchwork.The Masonite was screwed directly onto the supporting frame.

Brick walls. Masonite was screwed directly onto the walls using masonry wall plugs. As I continued the plywood topped benchwork right up to the walls, the Masonite was easier to install by allowing it to rest on the top of the plywood.

For both of the above methods, the screws were covered with drywall mud, joints were taped and also covered with the mud. As per the other guys above, joints were sanded, and then all of the backdrop was primed prior to painting.

Backdrop has now been in place for three years without any sign of cracking.

Room is behind my garage, beneath our house. As close as you could ever get to a basement in Australia! Our climate here is very mild, with few extreme temperature changes. I live at the same latitude in the Southern Hemisphere as Santa Barbara, California is in the North, so climate is very similar.

Hope that helps.

Lyndon.  Nice southwest US

Lyndon.  Nice southwest US theme layout!

I grew up in California and while I do miss it, I am happy to have a basement here in northern Virginia.

I'll be hunting for Masonite hopefully soon here as I work on the layout.

Construction Adhesive and Flex Paste

I use a combination of 1/2" drywall and 1/8" birch plywood.  The seams are joined with mesh tape and a thin layer of construction adhesive embedded in it. Drywall compound goes on top of that.  It's been three to five years now, depending on the area of the basement.  Nothing moves.

My friend Miles Hale uses Woodland Scenics Flex Paste in his Masonite joints.

which side out?

For masonite backdrops, is it better to use the glossy-side out or the rough side? 

The oily baked side takes everything less well (tape, mud, paint) but is a lot smoother to start with...

Part of my layout (the parts against the walls) will have backdrop, but parts set against the floor-to-ceiling windows will not because I'd rather see out the windows. I may use masonite for the backdrops or just paint the wall behind the layout blue.

Sand it

I sand it lightly with 220 grit before gluing or painting.

Thanks everyone!

Thanks everyone! I'll take a crack at this in the next month or so. Wish me luck.