MIMF

I think I have a pretty good handle on most aspects of archtop guitar building at this point, but I’m still struggling with one step: carving a smooth recurve. I mean, I can do it, I eventually get there, but it’s a long and tortuous process, with hours spent correcting problems with tearout of wood fibers, dimples, bumps, dips and scratches. I’ve tried curved scrapers, finger planes and sanding blocks of all kinds, and generally use a combination of all three, but the whole process seems terribly inefficient to me. The cross-grain areas are of course the most problematic. I don't have problems like this with the rest of the carving. Is this just how it is, or am I doing something wrong? Any suggestions?

I'm tempted to suggest fabricating sandpaper blocks with left and right hand curves that match the rim radius and the recurve to keep tearout to a minimum. You'll need to find a fairly "stretchy" sandpaper to comply with the compound curve. 3M makes a purple paper that's impregnated with latex on the back that's a little sticky but also stretchy.
I tend to use Mirka abaranet for heavy sanding jobs because it doesn't get clogged and cuts extremely fast. Mirka sells 2.75" x 5" precut rectangles in packs of ten if you don't want to commit to a 10 yard roll. You might find the 5" or 6" discs even handier as they come in sampler packs with all the grits.
You can buy the hook sheets with adhesive backing from Klingspor's woodworking shop: https://www.woodworkingshop.com/product/vc12040/ I can't say if this material will easily comply to your compound curves but it's not indispensable to the operation.

David, thanks for the abranet suggestion. I'm going to try that. I've found that wine corks make pretty good sanding block for this purpose - and of course I have lots of those around. But nothing works perfectly, because the shape and radius of the recurve changes as you go around the guitar, at least on my instruments. It's much wider at the lower bout than at the waist.

Sanding blocks larger than a wine cork would help you to establish a level surface. I would take David's suggestions verbatim.

I tend to do the recurve over quite a long period of time. I start to rough in before the box is closed, to establish the thickness (plus .030" - .050") with a toothed blade finger plane so I get grooves but no tearout. I scrape the grooves away, and close the box. I finalise the edge of the top with long sanding blocks so it's a true plane, then I very gently use a random orbit sander with a 5" diameter disc, at an oblique angle, to fair the curve of the recurve and blend into the top arch. I do both the top and the back to this point, then I start the whole tapping deal. I listen to the tone, I feel for vibration, I basically just play it like a drum, I listen to the free air pitch of the box. I iterate between scraper and sanding until I feel like stopping. At this point the outer 1/4" or a little more is still flat, and I route for binding, and install. The final carve is part of leveling and scraping the binding flush, I also bring the recurve out to a well faired blend to the binding. Then I finish sand. Takes me for bloody ever. I take at least 6 months to build a guitar. Part of my process is doing some things, like the recurve, very slowly because I tend to not be able to see things when I have been at a task for a little while. I have to leave it until tomorrow and look at it with no pre-conceptions and with fresh eyes and different light. Necks are the same thing. I CAN carve a neck in about two hours, but I take several days. Every single time I come back to it, I find things I want to change. I stop when the things get really really small...

Edit: - I often spray a coat or two of shellac to get a gloss, it really helps bring out the contours of the curves.

Edit two: - It's important, while still working on the free plates, to remember that the recurve is actually two sides of the same coin - you have to establish the back of the recurve on the inside of the plate, particularly where it fairs into the arch of the middle of the top.

Brian Evans wrote:Takes me for bloody ever.

I'm thinking that's the inescapable truth. Thanks for the comments.

By the way, I picked up some sheets of that Abranet stuff Dave King recommended, and I love the fine grits for sanding back shellac. It loads up, but then you can just flick the dust off, or vacuum it off with one swipe, and keep going. Great stuff.

I have quite a bit of the Mirka discs, but I always reach for the Abranet now, as it tends to leave less swirl marks than the paper.

For my sanding blocks I like to use the Russian/Euro birch plywood as the thin layers help me see how fair the curves are. I then line the blocks with a thin layer (about 1-2mm) of firm closed cell foam that can also be sanded and I stick that in place with either double-sided tape or Barge cement.

Last time I shaped a spruce archtop plate I was having difficulty getting a smooth recurve so I made a sanding block from a 4" long piece of a 2x4. I shaped a nice curve on one of the flat faces and put some 150 grit paper on it. I was shocked how quickly I got a nice smooth recurve with no valleys or mountains. It was sort of automatic. There is something to be said for larger sanding blocks.

OK guys, I'm going to try the larger sanding blocks on my next build - thanks for the suggestions.