Caulk

Dave, with inside corner joints where you use the L for backing reinforcement, do you see any reason not to run a bead of caulk down the inside corner before painting, presuming a permanent installation? It would seem to clean up the corner a lot, but maybe it’s excess fussiness when removability is more important.

Corner

If the gap is objectionable, you can run a bead of caulk.

Or you can use inside corner trim strip.

Or you can put cove molding in the corner.

I have relatively few inside corners so its not a big issue, but if the "raw" corner bothers you , there are several ways to hide it.

maybe it’s excess fussiness when removability is more important

If you think you may want to modify the layout it might be worth using screws and removable trim. I glued and air nailed my fascia for ease of installation but today I had to pry and scrape about 10 feet of it off to modify the track plan so screws don't seem so bad :> ) ....DaveB

Masking tape

Ive had good success with a strip of 1" masking tape over the joint, then once painted the joint almost disappears.

A clean cut or sand off the fuzzy edges also makes it smooth out better.

I have also used 2" wide tape for a corner joint.

@ Dave1905

good idea on the paneling/shower enclosure trim pieces, nice finished look. I hadn't thought of that one. I ended up using Evergreen styrene "H" pieces to fill the seam. They were a nice snug fit, so no adhesive needed.  Randy above mentions masking tape. Interesting! Might have to experiment with that one.  It took a little bit of trial & error with the circular saw on some scrap hard board, scribing it so it would bend without cracking/breaking, to make the radiuses.  Too shallow a cut & the board wouldn't bend to the radius. Too deep a cut & it would crack.  Finish washers give a nice look, only difference there, I used oval head screws... Seam in center....

Wood filler

On my last layout, I used 1/4” MDF for fascia.  I made sure the ends were square, butted them up against each other with a backer block behind, and glued the whole mess together.  Then used wood filler  to clean up any remaining gaps, and sanded it flush.  The result looked pretty good.

Getting the tight radius corners with 1/4” MDF was interesting, but not too hard.  I wet it thoroughly, bent it around the curve, and clamped it.  Broke one piece due to not getting it wet enough, but the rest went well.

More details in this blog post:  https://rices-rails.blogspot.com/2011/11/palmer-industrial-fascia.html

I haven't done it, but Randy

I haven't done it, but Randy Sellers used 409 to bend his hardboard fascia instead of kerfing the back. Should be much easier. But water to bend MDF looks even easier than 409...

If vinyl is so much easier than hardboard to use for backdrops, why don't people use it more for fascia? Too flexible?

Gotta admit

there's been a few times I would have liked to remove a piece of facia without using a saw. But on the other hand I like a real clean surface to set off the rather rugged surface of the layout. The room is less than 8' wide so the bends have to be very severe. Backdrop, facia and valance are all 1/8" hardboard without any kerfs or soaking. The joints are in straight areas with extensive backing and are sanded down, glued and screwed, patched and a final sanding. Sounds like overkill but it's actually quite easy.

bending masonite?

If wetting it doesn't soften it enough try boiling the water. Works great on green wood but never tried it  on masonite. I guess it would either help the bend or melt the masonite :> )  I've steamed ribs( oak not pork) for small boats in a long pipe propped up at an angle with a fire under the bottom ......DaveB

That's what I

wanted to ask Ken about, using warm/hot water to bend the masonite. Did that make a difference? 

To tell ya,  I had thought that water and a wood product don't mix, so I'll. kerf the stuff. (I am assuming its a wood product,) Rick mentions 1/8 hardboard, Ken is dealing with 1/4 MDF. Alright, pretty easy to do the math there. I've always called this stuff masonite. Just so I'm on the same page, are we talking about the same thing?  I found the 1/8 inch stuff and when I did the two corners in the pic, I took a test piece and tried pushing into the corner, I couldn't get it there without breaking it. Rick, I understand you anchoring the ends down,  any idea off the tightest radius you've used? 

Bending MDF

I used cold water, I didn’t try hot.  My approach was to soak it thoroughly only in the place I wanted the bend.  The way I made that work was by wrapping a towel around it over a shallow container (trash can lid was handy) and keeping the towell soaked by going over and pouring water over it every few minutes while I was working on something else.  I bent it a little bit at a time by propping up the two ends and pushing them together gently.  As it got more and more soaked, the bend progressed.  When it was close to 90 degrees I pulled it out and clamped it onto the benchwork so that it would dry in the shape I wanted.  It worked.  There was no damage to the integrity of the MDF that I could see.  Once dried it still felt as strong as the unbent sections, and the surface was only a tiny bit rough - if it was a board you’d have called it raised grain.  The whole process took a while, I think it may have been a couple/three hours but I don’t really remember for sure.

I tried to get two bends in a single strip so I wouldn’t have to have a seam at the end of that peninsula, but getting the second bend in exactly the right place relative to the first during the soaking process was pretty hard, and the whole thing was way more awkward with a bend in one of the ends I was trying to use to get the second bend.  That’s the one I ended up breaking.  So I used two pieces and filled and sanded the joint at the end.

I started the experiment not sure if I’d end up with a pile of mush, a bunch of broken pieces, or something good.  I think it turned out pretty nicely.  I think the key was patience and lots of soaking.

Bending Masonite...

To bend Masonite use Formula 409.  Wet the area thoroughly with the 409, rub it in a bit and you will feel it relax as you are working with it.  You can bend it to a very tight radius with ease.  Works like magic. 

Randy

Masonite/Hard Board VS MDF

Masonite, some times called hardboard is not the same thing as MDF.

I was going to explain it myself, but found it already on Wikipedia:

Medium-density fibreboard (MDF) is an  engineered wood product made by breaking down hardwood or softwood residuals into  wood fibres, often in a  defibrator, combining it with  wax and a  resin binder, and forming it into panels by applying high  temperature and  pressure. ^[1] MDF is generally denser than  plywood. It is made up of separated fibres but can be used as a  building material similar in application to plywood. It is stronger and denser than  particle board. ^[2]

Masonite is a type of  hardboard, a kind of  engineered wood, which is made of steam-cooked and pressure-molded wood fibers in a process patented by William H. Mason. ^[2] It is also called Quartrboard, ^[3] Isorel, hernit, karlit, torex, treetex, ^[4] and pressboard.

Masonite is formed using the Mason method, ^[10] in which  wood chips are disintegrated by saturating them with 100-pound-per-square-inch (690 kPa) steam, then increasing the steam or air pressure to 400 pounds per square inch (2,800 kPa) and suddenly releasing them through an orifice to atmospheric pressure. Forming the fibers into boards on a screen, the boards are then pressed and heated to form the finished product with a smooth burnished finish. (Later a dry process with two burnished surfaces was also used.) The original  lignin in the wood serves to bond the fibers without any added  adhesive. The long fibers give Masonite a high  bending strength,  tensile strength, density, and stability. Unlike other composite wood panels, no  formaldehyde-based resins are used to bind the fibers in Masonite

End of Wikipedia definitions.

MDF and Masonite come in both 1/8 and 1/4 thickness, MDF goes on up, to at least 1 inch, but for the common 4x8 foot sheet its 1/4 - 3/4 inch for MDF and 1/8 - 1/4 inch for Masonite.

I have also seen Masonite in tempered and non tempered versions, at least labeled that way and the tempered version did seem stiffer.

I would say bending either with water requires you to watch that it doesn't soak too long as they both will turn to mush, leave a piece out and the rain and see what happens.

How tight is the bend?

I took a tape measure and checked the bends on the fascia. The tightest one is a 12" radius. Others are in the 14"-18" range. The tight one is on the right looking at the bridge.

@ Jim B

Good information.  To be sure,  calling this stuff that I have, slick side, rough side, Masonite, (like the peg board material,) is that right?

Ken, does the MDF have two different surfaces?   

MDF

@Jerry, both surfaces on MDF are the same.

Masonite

Yes Masonite does have a smooth/slick side and a rough side, peg board material is Masonite. I would say from reading the description of how Masonite is made I now know why sometimes the back rough side looks like it was pushed against a window screen.

tempered vs untempered masonite - my 2 cents

Masonite can be 'tempered'. That creates a side that is harder, smoother, and almost oily. Untempered masonite has a slightly rough texture. Pegboard (basically just masonite with regular-spaced holes) can be either tempered or not (long time ago we used to have a little of both floating around in my Dad's shop - so maybe today it's all one way or the other, but there's no particular reason I'm aware of why pegboard must be tempered)

I think I read that the tempered side is better for painting because it's smoother. I'm not sure what you have to do to get it ready to paint so the paint sticks to it, it seems to me the oil residue would be an issue.

The tempered side is not so good for gluing, because the glue doesn't want to permeate the masonite through that side.

You might find that the masonite at the store has both sides tempered, or just one. I think it's supposed to be difficult to find masonite that hasn't been tempered on either side, but that's what would be best for making splines because of the glue issue. For backdrops and fascia you'd want a tempered side facing out.

If you're going to drive nails into it, I think tempered will be more difficult to use without pre-drilling holes.

Untempered

I prefer untempered 1/8 hardboard for fascia.  I can cut it easily with a utility knife.  The tempered material is much harder to cut.

oil residue

 Handling the Masonite, it never occurred to me that there could be some kind of residue. I mounted it with the smooth side out and as you can see from the pic I painted it. (latex house trim) It didn't hesitate adhering to it. I suppose something to be aware of.......

Tempered, untempered, didn't know that until this tread,,,,,

Residue

I have only had one sheet of Masonite (tempered) that had a problem with oil bleeding through the paint.  In most cases I have to put 3 coats of paint on the Masonite (or a primer and two coats) just because of the dark color.  The tempered has a much smoother surface, some of the untempered can have a "fiberous" texture when painted.