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Polyurethane Repair

Discussion in 'Luthier's Corner' started by ddnidd1, Aug 25, 2005.

  1. ddnidd1

    ddnidd1 Supporting Member

    I have a Moon JJ-5 bass that has a Thick, clear gloss finish that I Assume is polyurethane. The previous owner apparently tried to Mickey Mouse a truss rod adjustment.

    I've included an image of the dent I'd like to repair. However, I don't want it to appear that I put a noticeable glob of some clear material on it. And yes I am a bit of a fanatic about keeping my basses in mint condition.

    My first question is - can it be anything else other than polyurethane, considering the thickness? Next what is the best approach to repairing this so hopefully the repair is essentially invisible.

    I'm good mechanically and with tools. I regularly remove scratches in finishes, etc. so I'm confortable executing the repair. I just need to know the best procedure from someone with experience in this area.

    Thanks in advance for any responses.

    Attached Files:

  2. I will not comment on the methods for repairing, I'll leave that to the more experienced in repairing finishes. However, I can comment that it can certainly be stuff other than polyurethane. It can be lacquer, acrylic and a number of other finishes. I don't know what the Moon guys use.
  3. I do this sort of repair all the time and I use CA for it. Liquid is preferred but gel will work in a pinch. The liquid just seems to flow out clearer. Here's how I would approach the repair...

    First, I would tape off the area at least 1" around the scar. You can use masking tape. I use my own "luthiers paper" invention that's a little tougher but masking tape will work. I would tape over the dinger then use an exacto to cut out the area just a little larger than the chip. Next, I would use a small brush to wipe some naptha in the chip to clean it of any chips or goofy gunk that has gotten in there...but if you're as attentive to your instruments as you claim, that's probably not a problem. Now it's time to start applying the CA. Most bottles don't have an applicator tip small enough to do something like this cleanly so you might want to consider either getting a little applicator tube from the hobby store OR dripping the glue down a straight pin into the chip. One drop should go in the chip and then us the pin to push the CA around and under the lip of any of the clear coat that has lifted around the edges. Let this first drop dry in place. That might take 15 or 20 minutes depending on humidity. After that first application is solid, do it again. Now, here's where the impatient part with rest of us. You CAN just fill up the chip with CA by putting drop after drop in the cavity and letting the bubble of glue swell up above the surrounding surface. If you do, you'll have to let it sit overnight to cure. When you return, the CA will have cured enough to begin leveling. If you are patient with the procedure, you can put single drops in the chip and let each on dry on top the other and it will go faster. Just don't touch them to test their cure - use the straight pin to poke it and see if the repair is soft. If it is just wait.

    Once the CA has filled the chip and hardened, you begin leveling it. I use some 320 grit on a small wood block and just swirl it around over the chip, being careful to avoid scarring up my masking too much. Don't sand a large area, just do small circles. As the mound of CA goes down it will get flatter until it's as flat and as thin as the tape that surrounds the repair. Here I make a decision. If the instrument is a dark color, I'll leave the tape on and continue with 600 grit paper wet until I feel that the repair is leveled well enough. If the repair is something like you've got, I will take the mask off and level without it. The reason? The darker colors show up sandpaper scratches more and I want to minimize the area in which they occur. With the natural finish, you won't see them after buffing so it doesn't matter. The 600 wet sanding should get you down even with the top surface. Once that's level and smooth, I use 1500 grit wet to polish the top and then I use polishing compound to put the gloss back in the surface. A little paperbag polishing doesn't hurt either. After I'm satisfied with the polish, I'll wax it and then buff to whatever sheen I like.

    Done correctly, you won't be able to see this repair. The clear CA welds up all of the Poly and looks like what surrounds it. If you can get it good and polished, it will look great.
  4. ddnidd1

    ddnidd1 Supporting Member

    Thanks much for the detailed explanation. It's just what I was looking for. Is any particular brand/type of CA (other than what you mentioned) recommended? Also, what is paperbag polishing? That's a new one to me.
  5. You can use any for this type of thing.

    A brown paper bag is the equivalent of about 2400 grit sandpaper - especially the ones from the grocery stores. Try it on something the next time you think you've gotten it as smooth or as polished as you think you can.
  6. ddnidd1

    ddnidd1 Supporting Member

    I just tried it. Son of a b**ch - that's amazing!
  7. dpmasunder


    Apr 30, 2005
    I'll have to try that paper bag trick, Hambone.
    CA repairs are generally very good, recently we procured a dentist friends old blue light and are doing dental filling repairs and that kind of thing. Clear light cure dental lacquer is very nice to work with.
    I use 3m papers now which cut amazingly quickly and smoothly, levelling with 1500 is not a problem, then 2000, usually followed by Finesse-It and 2 grades of Tamiya polishing compound, depending on the gloss to be matched.
    But, for a situation like that particular one, I'd do a CA fill, using ultra thin to get right under the original finish, then a thicker CA fill. Same buffing procedure regardless.
  8. marsk


    Aug 17, 2002
    I would like to mention that modern glossy finishes are actually more likely to be polyester, which is different from polyurethane, and if you would like to repair the finish in a way that matches the original, it is possible to obtain polyester repair kits. Plan on spending a couple of hundred dollars for the kit; while a polyester product for repair is available in tube form (costing $20-$30), it doesn't achieve the same high gloss effect as a proper poly repair. There is quite a bit of technique involved in polyester repair as well, so if you decide to get into it, it would be helpful to start with a practice piece that may have a flaw in an inconspicuous place , such as the inner facing surface of a piano bench leg.
  9. Faced with such a repair and no prior knowledge, how would one determine if a finish were an industrial 2 part catalyst hardened polyurethane or the same quality of polyester. I was just in the Roscoe shop and I saw quantities of both. :confused: :meh:
  10. ddnidd1

    ddnidd1 Supporting Member

    Assuming that the finish is not polyurethane, but some other undetermined finish, how badly can I screw things up if I proceed with the CA repair? This is a Moon JJ-5 bass from Japan so I doubt I can get information from the company re: the finish. I'll try but I'm not expecting much.
  11. Lets wait for a reply but this question is at the heart of the matter isn't it? For a small repair like this, how would one tell what it was repaired with when one couldn't see the repair and if one didn't know what either the compositioin was of the original finish or the repair?
  12. dpmasunder


    Apr 30, 2005
    Quick question. Is that a high gloss finish, or slightly satin?
  13. ddnidd1

    ddnidd1 Supporting Member

    It's a very high gloss finish. It's actually the best high gloss finish I've ever seen on a bass.