Refinishing a guitar always seemed like a daunting prospect, requiring saintly patience and too much rubbing away with multiple grades of super fine sandpaper, plus a buffer and various combinations of chemicals to make it shiny at the end. After working with polyurethane on some projects around the house, I was pretty sure I could avoid all this and still create a beautiful, gleaming finish. I was correct! Here’s how.
Save time and heartache: learn from my mistakes where highlighted! Learn too from my knowledge-trawls through the manly, pipe smoking world of the wood forums, seeking technique and tips.
What is Polyurethane? A ubiquitous finish that can be sprayed, wiped or brushed, developed for slick shiny floors. The wood forum people look down on it and on every occasion implore you to buy other finishes which are not for sale anywhere. Poly has some annoying properties, but is cheap, hard-wearing, hella shiny, and available. The big home improvement stores carry poly, and everything else you’ll need:
- Rubber gloves
- Sandpaper: 60 and 220 regular, 400 and 600 ‘wet or dry’
- Spray bottle
- Wipe-On Poly (1 can)
- Mineral spirits
The stuff you see in round cans is thick, gloopy, applied with a brush. This builds a finish quickly on something flat like a floor or tabletop, but gets belligerent when met with the curves and contours of a guitar body.
Don’t be me: I carefully applied poly to the guitar with a natural bristle brush. It’s too thick and the brush holds too much. You get runs all around the edges that are much more troublesome than just doing it right.
Wipe-On Poly is a thinner concoction, thin enough to be wiped onto the wood without leaving trails. Wood forum people will tell you that, if you must use it, you can save money by mixing your own, a simple ratio of 1:1 polyurethane to mineral spirits. If you’re doing kitchen cabinets or a set of furniture, this saving likely adds up. For a small project like a guitar, I’m happy to pay the $15 for a can of premixed.
Strip and Prep
1. Sand off the original finish with heavy grit paper. 60 or 80 will charge through the thick layers of paint and clearcoat with gratifying speed. No tools are required for this tutorial, but if you do have a sander, break it out; it will work on the flat areas. Wear a dust mask, eye protection, and gloves. Expect blisters. Wear clothes of which you are not fond. It need not take forever with this coarse paper. You’ll see results quickly.
Stop often to slap caked-up finish off the paper, and to test how rough it is versus a fresh piece. Swap out if worn. Paper is cheap, and with worn stuff you’re working too hard for too little effect. Take care once through the finish; you can easily reshape raw wood with such rough paper.
2. When the sweating and the swearing are over and you have a naked wooden guitar, sand the whole thing smooth with 220, going with the grain; that is, in straight up and down strokes parallel to the direction of the wood grain.
3. Wipe the body clean of sanding dust using a rag dampened with mineral spirits. Why not water? Polyurethane is an oil-based finish, and they don’t get on. Water under a poly finish makes cloudy white blotches. You’ll use a rag like this often. Keep it handy.
4. With masking tape or blue painter’s tape, tape off any exposed areas that should not have finish on them, like inside the neck pocket.
5. Hang the guitar up where you’ll be working on it, either by running string or picture-hanging wire through one of the holes in the neck pocket, or by screwing a length of scrap wood to the neck pocket, using those same holes, and hanging that up. I did both at various times, and the scrap wood method works best because it gives you a safe kind of handle to steady or rotate the body when applying finish.
Color and Base
1. Stain now, if you wish, following the guidelines on the can. Test on some scrap wood to get an idea of the color, though it’ll warm up quite a bit under the amber hue of the forthcoming poly. I experimented with mixing danish oil colors until arriving at the burnt sugar combination shown. If you’re not a projects person with scrap wood lying around, buy a 2×4 and have it chopped up at the store. If using Danish oil under poly like this, wood forum wisdom suggests waiting a week until the oil is truly dry before proceeding with the poly. If not, it can never dry, and will keep any finish on top from drying, too.
2. Seal the wood. The first thing you have to get down is a sealing coat, a base for all the others to build on. It’s not pretty, it’s about stopping up the wood so that your later coats don’t get sucked into the grain. Saturate the surface with the polyurethane on a rag, using the reflection of a light or window to see which areas drink up the finish, and redo these spots as often as needed over about ten minutes. This is the only time you get to go back and touch areas like this during a coat, because it always comes out rough. Let that all harden for a day and you’re ready to really get finishing.
Building a Finish
Here is your life for the next while:
1. Any bad runs from the previous coat, sand down with your 220, though you’ll soon get out of the habit of causing these. For smaller imperfections, use black ‘wet or dry’ labeled 400 grit sandpaper, soaked overnight in a bucket of water. Take your $1 spray bottle, loaded with water and a dash of soap, and frequently spray down the areas you’re working on. This is wet sanding. Return the paper to the bucket often, washing off the white sludge. With the flaws addressed, get the whole body back to peach smooth before the next coat by wet sanding with 600. Wipe clean with your mineral spirits rag.
The key to repeatability is not making any of this a big deal. This is not your sweating, grunting kind of sanding. Don’t press hard. The weight of your hand is enough, and you’ll just leave deeper scratches. Go especially lightly at the corners, where the top and back meet the sides. There’s a lot less finish here and you’ll go through in a heartbeat. Glance over, move on.
2. Working directly from the can of wipe-on poly is tricky, so give it a good shake, then pour a decent whiskey measure into a clean jam jar. Be sure to replace the lid between coats.
3. Use a fresh rag each time. Cut up anything soft and cotton into three inch wide strips you can bundle up. Take one, flap it around to shake off the worst of the lint, wad it up and dip it into the poly. Less lint ends up in your finish if you wad up the rag in such a way that all raw cut edges are gathered up away from the guitar’s surface. Press out the excess against the lip of the jar. Dripping wet is too much.
4. Now let’s get it on the body. The best tip from the wood forum knowledge trove is to apply poly like the busboy at the diner. He doesn’t get all intense about it, extending a little finger and drawing the cloth across the table in slow parallel lines. He takes a damp, not wet rag, quickly and without ceremony wipes down the surface, and is gone before you read his name tag. Be the busboy. Wipe it down, get out. Poly hates to be reworked. Go back to try and fix something and it’ll set in an ugly way. You won’t believe this and will try it anyway.
Warning: Lay the rag out flat to dry, not scrunched up, outdoors if possible. Poly makes heat as it dries, and if that heat is unable to escape, because you threw the rag in the trash or balled it up on the floor, it can start a fire. Once dry, it’s safe to toss.
5. In a few hours, depending on heat and humidity, the body can be ready for the next coat. How do you know? With the tip of a little finger, rub lightly at a spot that will ultimately be hidden, such as under a pickguard or neck plate. If it’s not tacky, repeat the sanding and poly steps. You can get four coats done in a day. If you’re ever sanding between coats and notice little elliptical balls of finish like grains of rice, it’s not hard enough yet and you’re going to make a mess. Abort. Come back later.
Don’t be me: Poly doesn’t stick to itself well and wants to run. It really wants to. If you get impatient and try to put on any more than the minimum, you’ll get runs that take much more time and effort to fix than just building up quick, light coats.
1. The finish gets deeper and glossier the more coats you add. Once you’ve reached your desired look, the trick to getting out without all those steps of dispiriting multi-grade sanding lies in the final two coats. By now you’ll have a feel for the right amount of finish to take into the rag without causing problems. First you need to put down the perfect coat: no runs, sags, dry streaks or rough spots; nothing that needs to be sanded out with your 220 or 400 paper, as the next coat is never enough to hide the scratches. One more ultra light, baby soft wet sand with the 600, giving the body a smooth satin texture for the poly to stick to, a thorough clean up with mineral spirits, and wipe on one last blessed coat.
2. Relax. For the last part, the finish has to be not just dry, but cured, which takes weeks. Leave the body hanging, and sniff it once in a while. If it still smells of polyurethane, it’s still curing.
3. The last trick from the wood people is a good one. Once cured, “sand” the piece with a brown paper grocery bag, which it’s said is akin to 2500 grit. Hit it with 3M Scratch Remover, following the bottle’s directions, pressing firmly in circular motions until the cloth squeaks, and watch a glassy finish emerge to reflect the room around you. Rejoice.