 | |
|
Plenty of sanding tasks in this business need to be done by hand and making your own specialized blocks can help. Most of the blocks in the JET Guitars shop are made from scrap korina or mahogany, both of which are easy to shape on a 48” belt sander.
The photo shows some of the various sanding blocks used in our shop, starting with the basic flat block (left). Moving to the right, the radiused blocks are indispensable for neck and fingerboard sanding. Most of these have fine sandpaper glued onto them to help grip your sanding sheet, although some use a leather lining for fine sanding. A stick with a French curve profile is useful for sanding chamfers. A softer foam block for final scratch removal tasks on fingerboards, rims or during clearcoat rubout frequently comes in handy. Finally, the Corian wedge (right) is used in corners during rubout. Since this is a wet process, Corian is better than wood because it won’t warp and will retain a sharp edge.
In my shop, a lot of the sanding is done with 1/4” sheets and cut-up belts that have been used on the machines. As far as brands go, I have had good experiences with Canada’s Carborundum brand and 3M. If you want a really fast-cutting and long-lasting product, I recommend 3M SandBlaster maroon 100 and 150 grit. When you get down to 220, using aluminum oxide (AO) is perfectly acceptable, although some prefer garnet for the final step. That’s the overview for wood sanding. Wet sanding of the final clearcoat prior to buffing – rubout – is a completely different animal. A 5” PSA air sander, running at around 50 psi, is my tool of choice for larger surfaces. For smaller surfaces like the headstock, I like to slow down the sander. I get good results using Klingspor silicon carbide discs (particularly PS11/12700T). Plan on using one disc of each grit per guitar and approximately three hours for this step. Runs, edge build and drop fills should be leveled using a scraper or microscope slide before you start sanding.
The basic technique is to soak the disc in water, then sand at moderate speed. Start with 320 or 400 grit, and step through 600, 800, and finally 1000. You will need to frequently wipe off the residue and water, and inspect the surface – look for pocks, scratches and swirls, and go over them until they are gone. Being thorough at this stage will pay off big because flaws that are hard to see during rubout always appear during buffing. In fact, I almost always find some swirl or scratch during buffing that needs a little more 1000 grit hand sanding.
A major risk during rubout is burning right through to the wood, which will require you to stop and repair the area. You may also see lifting of the finish if water gets into the wood near holes and cavities, although this may be reversible if it’s not too severe. Learning how to prepare and head off these problems is a completely different process that begins even before painting, and it is important to note that nitrocellulose, urethane and polyester are all chemically different coatings, each requiring its own mindset and touch during rubout and buffing. If you’re used to the heavy, hard, dry character of polyester, you’ll need to lighten up a lot and get used to more rolling and clogging of the abrasive disc as you switch to nitro, for example.
Curved areas like the neck and rims need to be rubbed out by hand – there is no way I would trust a machine on them. You could use a soft block, but I usually rely on my hands. In my shop the used SC discs are recycled for fret polishing or other metal work.
Jeffrey Earle T.
Jeffrey Earle T. handbuilds JET Guitars in North Carolina, USA.
jetguitars.com
Abrasive products and machines are essential to every facet of luthiery, from wood sanding to finish rubout. This two-part column will give you a rundown on the abrasives used in the JET Guitars shop, and valuable advice about using them. As you will see, all of the abrasive products are cycled through two or more “life stages” to get the most out of them before throwing them away. The basic tools used with them are a 48” x 6” belt sander, a 1/4 sheet electric random-orbit palm sander, a variety of sanding blocks and a 5” PSA air sander.
The belt sander is used for thickness sanding and rough shaping work. I use 60 and 100 grit aluminum oxide belts – I’m not too picky about the brand – with 150 and 180 grit for medium sanding. A Teflon platen is a good feature to reduce friction, and when it starts cupping, you can true it flat again or replace it. You might want to wrap the drums with friction tape – used on baseball bats – to eliminate the annoying drop-off from the platen. Another feature to look for is a quick-change lever. You will most likely need to adjust it so it works right, and maybe modify the arm to keep it out of the way. The most essential component of a machine like this is the quality of the bearings. Unfortunately it’s hard to know how good they are until years later when they start to squeal and wobble. Sanding belts can be recycled into metal work, or cut up for hand sanding after they start wearing out.
The electric palm sander, loaded with 180 and 220 grit AO, is for fine sanding of maple. I also use it to sand out the first stain wash on a figured top (for deep grain enhancement) and I work the tool pretty hard for that task. Using a lot of pressure goes against most recommendations for sanders like this, but hey, I’m a bad boy – and it works. You can deal with carved top re-curves by changing the stock pad with a thicker one that has rounder sides (see photo). Naturally, that is something you will never find for sale, so you will have to fabricate or modify one. Some luthiers like to use a disc sander on carved tops.
Selecting a random-orbit sander can be tricky, and you will go through more than one, but from my experience, here are the weak features to avoid when picking one out:
- Insufficient power
- Flimsy plastic internal support pylons that can break
- Wimpy paper fasteners that slip, break or are hard to use
- A sliding on/off switch, or one mounted on the sander’s handle
Always remember that the pad on this sander is not very firm. Fingerboards and headstock faces – especially with inlays – often require at least some precision hand sanding and a palm sander can distort their shape. Also, don’t use this tool on softer, plain-grained woods such as korina or alder as it leaves swirl marks that appear after staining. Instead, use a belt sander, and then hand sand with the grain using a block.
Used 180 and 220 grit quarter sheets get a second chance when it’s time to hand sand corners and roundover areas. Don’t throw them away yet! When they’re so beat up you think they can’t be good for anything else, keep a stack around for roughing clearcoats between spray coats.
In part two we’ll look at sanding blocks and air sanders.
Jeffrey Earle T.
Jeffrey Earle T. handbuilds JET Guitars in North Carolina, USA.
jetguitars.com
 | |
|
- They may be all you have room for, or can afford, if you’re just starting out.
- They give you a really intimate, organic understanding of the structure and behavior of wood fibers – which can give you experience to help you avoid large-scale errors (such as tearing
- For some tasks, there is simply no better tool than the one you have in your hand
Wood magazines and books will give you instructions on using and sharpening a scraper, and it’s good advice for general woodworking. However I usually stop at “stoning,” and don’t proceed with “turning the hook” or “burnishing.” The scraper requires stoning pretty frequently when I use it, but it has only taken seven years to wear away over an inch of metal. I think the hardness of my materials and the technique I use depend more on a fine square edge than a burnished hook. The technique involves pushing or pulling the scraper nearly flat across the surface, using the leading edge to get a very fine, continuous shaving cut. A hooked edge – the traditional scraper edge – won’t cut in that direction. It will if you stand the scraper higher on its trailing edge to hog away more material, but I use a separate tool for that kind of carving.
 | |
|
The second photo shows the same technique on a clearcoat finish. This won’t work well on softer coatings like nitro, as it tends to grab and pull the paint off, but on hard stuff like polyester it is a great way to level runs, edge build, orange peel and drop fills without having to worry about sanding through.
It is essential to use both hands when scraping like this – you can’t control the tool with only one hand. So you need to find a way to hold your work firmly… perhaps in a future column!
Jeffrey Earle T.
Jeffrey Earle T. handbuilds JET Guitars in North Carolina, USA.
jetguitars.com