Ok, it's your own fault since some of you mentors inspired me to try this...
The plan: Hammer veneer an MDF core with hide-glue and paper-backed commercially-purchased veneer (I already chickened out on shop-sawing).
I produced a band-sawn MDF core via the "cabriole leg" method (taping offcuts back on and re-re-sawing the 90* face). This creates at each corner the junction of two curved surfaces. Viewed straight-on, that intersection should be vertical line, and in my case there's a little undulation around vertical. I haven't tried any serious surfacing or hand-shaping yet, but that's my next step. My concern is that I'll keep chasing that line back and forth across the center, or in so doing ultimately end up with four dissimilar curved faces.
I'm content to have it at with the optimistic notion that it'll come out OK, but I'm wondering if those of you who've mastered this have any specific tips, guidance, "watch out for"'s. E.g. - it seems like the glue joints are so much harder than the surrounding MDF that it's possible to produce a "hump" when sanding. In fact, right off the BS it seems (altho I could be dreaming this) that whenever the blade encountered a glue joint at a very acute angle, the glue line wanted to divert the blade/ change its course a little.
I've ordered paper-backed quarter-sawn cherry veneer and I have a glue pot set up (hide glue in's the fridge right now), but I've never done any veneering, either. I've read a bunch of stuff online and it seems like this is the best (and most forgiving) approach to try. I remain unsure as to the purpose of the paper backing, and hence whether it's a pro- or con- for this job, but that's the only QS cherry veneer I could find.
1. make a paper template of the face surface, and use it to cut the veneer about 1/2" oversized in each direction. I understood (in advance this time!) that the curved surface will be longer than the "projection" - IOW, the shape of my 2D template won't work.
2. slather a coat of hide glue on the core, place the veneer, start spreading it out and hope for the best. I don't have experience with the time until it "gels," but I'll get when I practice on some scraps - I'll veneer the offcuts, maybe.
3. when that fails, I'll take it apart, let the hide glue dry, and try loosely taping the veneer with masking tape to hold it's position. Then I'll heat up a piece of steel rod I have and use that to iron (reactivate the glue) working either from the bottom up, top down, or middle out. Again, a few practice trials are in order.
4. when I successfully veneer the first face, I'll do the next face so the edge of the second veneer overlays the the glue-joint of the first. IOW, I'll trim one side of the veneer flush with the core, the other side flush with the previously-veneered face. Working my way around the core like this, that means there would only one side of that curved miter would show a veneer edge-joint. I'm hoping that it's unnoticeable, but maybe some super-careful hand work would allow me to put a bevel on that .042" edge so that joint disappears.
5. I've also toyed with the idea of using washcoat of shellac on the core first. Since the MDF surfaces exposed by the band-sawing are all "edge grain" the surface is pretty porous and cardboard-y. The thought is the shellac would secure that surface, and that hide glue will happily stick to dried shellac. If there are divots or low spots, I've even wondered if laying in some plaster of Paris to fill them would work. Again, this would give a super smooth (after sanding) surface and shellac would isolate it. Maybe I just love plaster - it's cool stuff.
Short of a vac bag set-up (which I obviously don't have and have no $ to get), I don't think there are any specialized tools I don't have or couldn't get to get this job done.
We wanted to respond with the "sculpting portion" of our reply first because we know you are working on this as we speak.
Depending on how well your band saw cut your core, the shaping can be accomplished with several tool options. You can start with anything from spokeshaves, to a second cut cabinetmaker's rasp, working through to sandpaper on a block. You really want to be careful of your four corner edges as the MDF can get gouged or nicked pretty easily, especially when using the coarser cutting tools.
Making a fixture to hold the core horizontal during the sculpting process can be a great advantage. You could use one of your side "off cuts" screwed to a base board to support the workpiece on its side. By adding a stop block on one end, and clamping from the other, it should support and hold the piece securely on your bench.
Even though there is no real grain to MDF, shaping from the high sides down into the cove is the best practice and helps accomplish a smooth surface. Once you get to the sanding portion of your shaping, use a block that is wider than the sculpted face and that should negate any problem with the glue lines.
A template of your desired curve made on the edge of a piece of MDF will help you get a consistency from face to face and allow you to monitor the high and low spots as you work around the core. Don't let the true-ness of your corner lines concern you. In fact, they are what can actually help you to perfect the shape. By carefully studying the line from a direct view, it tells you where you need to work. If it undulates to the right, then the face on the right is the one that needs further stock removal and that is the exact spot that needs to be taken down. You continue to carefully work and refine, until the corner line is straight.
Taking your time, use a template, let your edges be your guide, and you'll be fine!
If you have any questions about this, let us know, we'll be back with the veneer response later.
Thanks, John - and to the others behind the scenes.
I hadn't thought about using the offcuts as a jig to hold the work. Also - and I'm particularly not proud of this - I didn't even think of using the the template as a guide for checking the hand work. It's getting better and better.
we are a bit concerned about the paper backed veneer you purchased and the corner joinery. Is there any chance you could mock up a corner and glue two small pieces that butt? The concern is how the paper will affect the look in the corners as you will not be able to even soften the veneer in the corner without it exposing more paper.
We think your veneer methodology is very sound. We especially like that you are going to practice on some scrap first.
Your paper template is the perfect method to lay out and precut your veneer.
Each side of the pedestal presents a very nice size to work with. So rather than decide to "slather" on the hide glue,... brushing on a good even coat will work best.
You're going to find out that this is easier than you anticipate. As the hide glue begins to gel you will feel it start to "grab" the veneer, and that is exactly how hide glue works. As it cools and sets up, it shrinks and literally "grabs" every minute nook and cranny in the material.
You don't mention what you will use as a "hammer", but work from the center out as much as this shape will allow.
If you have problems, the heat set method of rescue is exactly what you do. The contour of your pedestal is gentle enough that a household iron would probably work better than your heated rod. I would place some butcher or kraft paper over the veneer so you don't scorch it, especially with cherry which burns easily.
Your application "rotation" described in number 4 is correct. BUT as far as your corner bevel and trying to hide the joint is where you must be very careful.
That brings us to the paper backed veneer. It is our experience that it is even thinner sliced than standard commercial 1/40" veneer and obviously more prone to sand through. It is usually sold by its total thickness including the paper. its PROS are that the paper structurally helps fragile burls and straight grained veneers that are prone to split. Its CONS are that it is very thin and paper backed!
So we recommend that when you practice on scrap, practice doing some corners too. Also remember that paper backed veneers are generally sanded to 220 grit from the factory so don't over do it on the flats either. If you can't make the corners look great consistently during practice, then by all means, let us help you find some regular quarter cherry veneer.
A true flitch of standard veneer would allow a better bookmatch too.
Lastly, you mention using the plaster as a filler. We would like to recommend Bondo over plaster, although either would work. We also think after very careful sculpting, you will wind up with a very well prepared MDF substrate and not need to do anything else to it.
Let us know if you have any more questions, and if we think of more we will add it in a follow up.
Again, I'd like to express my thanks to John and all the other mentors behind the scenes. I've been following the advice here, and I'm calling the core "done." There are areas where it's not completely perfect, but I think it will do its job and my lines are pretty straight.
Update: The veneer
I guess I should've expected as much (I didn't think about it) but the veneer I ordered is a little disappointing - the original flitches were only about 4-6" wide, so they book-matched 2, then slip-matched the book matches (if I'm getting my terminology correct). I have a 24" wide piece to work with, but I will only be able to get 2 7"-ish faces out of that width, and I'll have to center the bookmatch. So, I'll end up with a big seem right down the middle of each face (not my preference, but it'll have to do) and I'll have a lot of waste (I was hoping to have some left-overs out of this).
The paper itself is thicker and has more "hand", or body, or whatever it's called, than I expected. It's very stiff. On the plus side, it's very flat - even tho it came to me rolled up, it sprung back very flat upon leaving the box.
The trial run
I took my first stab at veneering on one of the offcuts, and I'll attach some terrible photography to prove it. Overall, I'm not unhappy with the results. In the photos, I'm zoomed way in trying to take super close-ups, so it doesn't look the greatest. Holding this thing a foot away from your eyes, it's more than passable to my meager standards, tho.
I brushed a coat of hide glue out on the MDF, layed the veneer, and immediately started squeegeeing with the corner of a block of cherry. The veneer just slid around. At some point, it started to gel and I thought I was out of the woods, but the ends weren't stuck. I peeled the whole thing off and to my surprise much of the paper backing wasn't even "wetted."
I brushed out more hide glue on the paper and tried again. Again, when the glue is still liquid, the hammering just slides the veneer around. As the hammer leaves a curved area, the veneer springs back up away from the core. When it starts to gel, it seems like whatever isn't in contact won't stick when pressure is applied. I found myself wishing I could clamp the ends down and work the field.
I got it as good as I could and had at the ends with an iron. It ultimately worked, but again, as soon as pressure is un-applied, liquid glue just lets the veneer spring back; gelling glue won't grab it. I ended up heating it again and just holding it down with my fingers. This is what ultimately worked, but I don't think it's a practical method for doing the large faces of the project (on this trial, I'm veneering both sides of a corner - the veneer is only about 3" wide and 10" long, and the curve is all convex).
It's not obvious how long it needs to set before working it further, but I started only a few minutes after I was done fiddling with the first face to start the second face; and only a few minutes after finishing the second face, I started trimming/ sanding to flush the corner.
The trimming/ sanding continually folded down a little "burr" of what I assume was the paper backing, altho I guess it could've been glue squeeze-out in some stage of curing. It was tedious but simple to trim/ scrape off most of that off with a razor blade, but again, the amount of time I spent futzing with a 10" long practice corner is not making me look forward to doing this on 8 larger faces (I'm making two of these tables).
OK, I realize that's quite a data dump, but I'm all ears.
My biggest issue right now is that actual hammering. It's not obvious to me (yet) when/ how the "grab" occurs, and at what point thereafter one ceases squeegeeing (or hammering, I guess I should say).