Alan W
Well-known member
This is an offshoot and a deeper dive off of a post I did in @BuddytheReow fabulous purple kit thread, into choosing what type of abrasive to use for finishing. The most common two choices are abrasive sheets and steel wool. The main difference between them is that a sheet of sandpaper will follow contours, etc. (assuming you don't have it mounted on a board) but it won't dip into fine recesses, like wood pores, which steel wool can. So—if you want a very smooth surface, sheet goods work better—but, if you want to have slight depressions where the grain pores are, the steel wool will get you there.
I think I've spent at least the required 10,000 hours sanding, but most of it was on pieces of acrylic or ABS, which are the main materials my shop fabricated in. At least 90% of the time, water was involved. Dry sanding is generally bad, for several reasons. First, it's not efficient. If you imagine a sheet of sandpaper hugely magnified, the peaks are the tops of the abrasive grit, and the valleys are the spaces between them. In no time at all, the spaces fill up with the dust you are making by sanding, so you lose a lot of efficiency—but a lubricant (water, in this case) keeps the sanding dust from building up, so your "peaks" stay higher than the "valleys," and sanding goes faster. Second, it truly is a lubricant—your paper will glide easier over the surface. Third, it greatly helps you avoid the buildup of little clumps of dust—which when you're sanding a paint finish, usually clump into small bumps (taller than grit) that are as hard as what you're sanding, and will leave big gouges into your surface. (This is about as frustrating as it gets; here you were almost finished and suddenly have to do a LOT more work.)
But, water's really not a good lubricant to use on wood, unless you're sure that there's a truly sealed surface, and one you're not going to sand through. So, for something like a guitar body or neck, I typically use mineral spirits. It doesn't take much. Note that only grey backed (sometimes other colors, but rarely brown or tan) "wet or dry" sheets can withstand water, or any other liquids—if I'm sanding down wood, (at least in my shop) I use an air compressor to blow the dust off the paper pretty often, to keep buildup down as much as possible.
TYPES OF SANDPAPER
I'm going to keep this really basic. For coarse abrasives, garnet, or a "production" paper, are less expensive, and work fine. This is typically 30 to 220 grit. In my practice, I rarely go over 120 with this type of paper, usually switching over to a "wet or dry" type. This is where it gets more complicated, but unless you're working with a lot of steel, just stay with paper backed sheets. (The next step up is cloth backing. There are also higher temp adhesives that can get used, but for hand sanding none of this matters.) The paper backing is also available in diferent weights—typically the finer the grit, the thinner the paper will be, but a complete line, like 3M has, will have several options in each grade.
As much as I hate to admit this, 3M is generally a good bit better than other brands I've tried (with Norton coming in a close second) — the material used to make the grit is harder, and sharper, and lasts longer. Over the years, I watched sleeves of 50 or 100 sheets start to cost several hundred $, and believe me, I tried cheaper papers—but they did not last or cut well enough to be cost effective. It's depressing to send in an $800 order for sandpaper for a small shop...
BACKINGS
If you're sanding something flat, then you want the sandpaper flat—for small parts, I'll mount the sandpaper on a board, and move the part over the board. For larger parts, I'll make a sanding block. For parts with contours, a lot of times it's fine to just use your hand holding the paper. But often, just having a slightly flexible layer, some added thickness, so that the sheet follows the contours, but the pressure from your fingers gets evened out, can really make a difference. At it's gentlest, this can be using spray adhesive to mount a thicker piece of paper behind the sandpaper, but I really like the 1 and 2 mm thick craft foam that "craft stores" carry for this purpose.
BACK TO GENERAL
Where you stop with grade depends on the finish you want to achieve. For matte finishes, there's rarely a need to go over 400. For a wet gloss though, 2500 my be about right. I can tell you this—if you want a nice gloss finish on something, make sure every coat has been smoothed out some. If I know I need a good gloss, I will make the primer have a sheen, usually by buffing it with soft paper towel. We're spray applying all our finishes, and with non-catalyzed clears, I figure about 3/4 of a thousandth inch for each coat. (My point being that it takes a lot of coats to begin to even out a surface that is not smooth—so make each surface as smooth as possible.) While you can probably paint something the size of a pedal without needing to do corrections on it, a guitar body will probably have a few areas that need some help—overspray, or (harder to deal with), drips. It's gotta be completely dry to do any work on it. After you go up through a few stages—if this was after the "final" clear coat, I'd probably start with 600 or 800, then 1200, and after 2000 or so, I'd use a polishing compound. (Make sure you get one that does not leave residue, usually marked by a statement that it can be finished over, or something like that.)
I haven't mentioned scotchbrite, or other sanding pads. These have pretty much replaced steel wool for commercial finishing, except for a few specialized areas. They are available in a selection of grades, but it may be hard to find more than the most common grades in stores.
Most cities will have auto paint suppliers. They may or may not be able to sell paints and solvents to non-industrial users (depends on state and local laws), but they will happily take your money for the abrasives and polishing compounds they sell. If you find your local hardware store has too limited a supply of sundries, that's a good place to look.
I think I've spent at least the required 10,000 hours sanding, but most of it was on pieces of acrylic or ABS, which are the main materials my shop fabricated in. At least 90% of the time, water was involved. Dry sanding is generally bad, for several reasons. First, it's not efficient. If you imagine a sheet of sandpaper hugely magnified, the peaks are the tops of the abrasive grit, and the valleys are the spaces between them. In no time at all, the spaces fill up with the dust you are making by sanding, so you lose a lot of efficiency—but a lubricant (water, in this case) keeps the sanding dust from building up, so your "peaks" stay higher than the "valleys," and sanding goes faster. Second, it truly is a lubricant—your paper will glide easier over the surface. Third, it greatly helps you avoid the buildup of little clumps of dust—which when you're sanding a paint finish, usually clump into small bumps (taller than grit) that are as hard as what you're sanding, and will leave big gouges into your surface. (This is about as frustrating as it gets; here you were almost finished and suddenly have to do a LOT more work.)
But, water's really not a good lubricant to use on wood, unless you're sure that there's a truly sealed surface, and one you're not going to sand through. So, for something like a guitar body or neck, I typically use mineral spirits. It doesn't take much. Note that only grey backed (sometimes other colors, but rarely brown or tan) "wet or dry" sheets can withstand water, or any other liquids—if I'm sanding down wood, (at least in my shop) I use an air compressor to blow the dust off the paper pretty often, to keep buildup down as much as possible.
TYPES OF SANDPAPER
I'm going to keep this really basic. For coarse abrasives, garnet, or a "production" paper, are less expensive, and work fine. This is typically 30 to 220 grit. In my practice, I rarely go over 120 with this type of paper, usually switching over to a "wet or dry" type. This is where it gets more complicated, but unless you're working with a lot of steel, just stay with paper backed sheets. (The next step up is cloth backing. There are also higher temp adhesives that can get used, but for hand sanding none of this matters.) The paper backing is also available in diferent weights—typically the finer the grit, the thinner the paper will be, but a complete line, like 3M has, will have several options in each grade.
As much as I hate to admit this, 3M is generally a good bit better than other brands I've tried (with Norton coming in a close second) — the material used to make the grit is harder, and sharper, and lasts longer. Over the years, I watched sleeves of 50 or 100 sheets start to cost several hundred $, and believe me, I tried cheaper papers—but they did not last or cut well enough to be cost effective. It's depressing to send in an $800 order for sandpaper for a small shop...
BACKINGS
If you're sanding something flat, then you want the sandpaper flat—for small parts, I'll mount the sandpaper on a board, and move the part over the board. For larger parts, I'll make a sanding block. For parts with contours, a lot of times it's fine to just use your hand holding the paper. But often, just having a slightly flexible layer, some added thickness, so that the sheet follows the contours, but the pressure from your fingers gets evened out, can really make a difference. At it's gentlest, this can be using spray adhesive to mount a thicker piece of paper behind the sandpaper, but I really like the 1 and 2 mm thick craft foam that "craft stores" carry for this purpose.
BACK TO GENERAL
Where you stop with grade depends on the finish you want to achieve. For matte finishes, there's rarely a need to go over 400. For a wet gloss though, 2500 my be about right. I can tell you this—if you want a nice gloss finish on something, make sure every coat has been smoothed out some. If I know I need a good gloss, I will make the primer have a sheen, usually by buffing it with soft paper towel. We're spray applying all our finishes, and with non-catalyzed clears, I figure about 3/4 of a thousandth inch for each coat. (My point being that it takes a lot of coats to begin to even out a surface that is not smooth—so make each surface as smooth as possible.) While you can probably paint something the size of a pedal without needing to do corrections on it, a guitar body will probably have a few areas that need some help—overspray, or (harder to deal with), drips. It's gotta be completely dry to do any work on it. After you go up through a few stages—if this was after the "final" clear coat, I'd probably start with 600 or 800, then 1200, and after 2000 or so, I'd use a polishing compound. (Make sure you get one that does not leave residue, usually marked by a statement that it can be finished over, or something like that.)
I haven't mentioned scotchbrite, or other sanding pads. These have pretty much replaced steel wool for commercial finishing, except for a few specialized areas. They are available in a selection of grades, but it may be hard to find more than the most common grades in stores.
Most cities will have auto paint suppliers. They may or may not be able to sell paints and solvents to non-industrial users (depends on state and local laws), but they will happily take your money for the abrasives and polishing compounds they sell. If you find your local hardware store has too limited a supply of sundries, that's a good place to look.
