Touching up finish

GregDunn · #1 09-09-2010, 12:35 PM

I'm sure this is a technique thing, but it's something I've been having trouble figuring out. I've gotten a few used instruments over the years, cleaned them up, and either kept or sold them according to the needs of the moment. A common problem is a scratch or gouge in the finish which is too deep to just polish out. So depending on the finish, I will apply some clear or color lacquer (not on nitro finishes, obviously - but I have never had one) and then polish down to mirror smooth.

Here's the problem: I know enough about this to be aware that you need to fill slightly higher than the surrounding surface and then polish level with it. But no matter how small a "tool" (be it cloth, sandpaper, or bare finger) I use, it always seems to take the surrounding surface down as well, and I end up with a raised fill and a dished surface around it, so it's not flat. Lather, rinse, repeat... either it takes hours to do a tiny area the size of my finger tip, or I get a contour instead of flat.

I use very fine sandpaper for the initial work (2000-3000 grit) and then work my way down with various grades of polishing compounds. I can get a nice smooth finish, but it's not flat - unless I spend way too much time on it, and I don't mean just a couple of hours per scratch.

Obviously I'm doing something wrong (I'm not a woodworker, so my basic training is missing

). Any suggestions from those who do know what they're doing, unlike me?

202dy · #2 09-09-2010, 01:41 PM

Scrape it.

The idea is to make the raised area level with the rest of the finish. This is a goal. How close you get to the goal is up to you and your hand skills.

How to make the scraper: Take a single edge razor blade and run the sharp edge on something hard. This action will turn a small J hook at the edge. It doesn't have to be huge, and the angle isn't critical. Some screw driver shafts are hard enough to do it. Some aren't. If you have one, a carbide rod is perfect.

Using the scraper: This is a two handed tool. Holding the scraper between the thumb and forefinger of both hands push or pull it over the raised (drop filled) area. A small, thin shaving of lacquer (CA glue, whatever) will come off the top of the lump. Continue until the lump is almost level with the surrounding surface. Then switch to a block that is roughly the size of the now very thin mound. Using water and some 320 grit and finish leveling the repair. If you did a good job of scraping you can start with 400 or maybe 600 grit. If you did not do so well, 220 grit. Or better yet, scrape a little more. Once it is completely level you can finishing rubbing out using grits to 2000. Then buff for a factory finish.

Notes: The hardened drop fill should be smooth before scraping. If it is not, a little work with a file will rectify the problem.

Some folks recommend wrapping a couple pieces of tape on either side of the razor blade as depth stops. If you use thin translucent auto tape, the drop fill will end up roughly .020" tall. That's a lot to try to sand through. However, it is a safe way to do it. If you use a naked blade, you may end up with a few light hairline scratches outside the drop fill area. If you are careful to keep them small, they will rub out.

Caveat: This is an advanced repair technique. If you are not handy in the extreme, or a single edge razor blade (not the thing you put in a utility knife) sounds like a blast from the past, or you think rubbing out is something done by guys in chalk strip suits, take the guitar to a pro.

GregDunn · #3 09-09-2010, 01:53 PM

Brilliant! That's something like I had in mind, but with many helpful details. I can certainly manage the technical aspects of that; it was the procedure I was getting hung up on. Many thanks.

SurferJoe46 · #4 09-10-2010, 09:31 AM

I use a nib file.

It won't cut the surrounding area and will only remove added finish that is higher than the rest of the original finish.

They cut in only one direction, so look at the nibs to see how they will cut first.

GregDunn · #5 09-10-2010, 02:13 PM

Is that something you must make yourself, or is it obtainable commercially? I confess I've never seen one of these before.

SurferJoe46 · #6 09-11-2010, 08:54 PM

No - they're available at pro auto-paint shops.

NIB FILE HERE

HOWEVER - their description is all wrong! IT itself is the file and you don't need any paper on it at all.

I can't believe they said what they did about it needing paper over it - gads! I imagine you COULD do that too - but that's not the designed purpose of it.

If you buy one - and it's best to go to an auto paint supplier - just look carefully at it and you'll see it's comprised of many small needles that are all bent and ground to one direction and it must NOT be a single piece of metal that has circle-cut grooves on it!

That one-piece design's just for shaving the blemishes.

This is the wrong type:::

The 'nib file' OTOH will remove any 'proud' areas that stick up over the base material and not cut any deeper or into the base material either.

This is the Real McCoy:::

GregDunn · #7 09-13-2010, 09:32 AM

Very cool - never knew of such a tool. I must own one.

202dy · #8 09-13-2010, 07:50 PM

The cut on the nib file is similar to the fret leveling files sold by Stew Mac. While a bit more expensive, the block on the three inch file is more user friendly and doubles for use in spot leveling.

SurferJoe46 · #9 09-13-2010, 07:54 PM

No - look again.

The true nib file is just that - NIBS that are stood on end and they are ground all in the same direction to ONLY cut something that is standing proud and will NOT cut the surrounding material.

A FILE - OTOH - will cut anything along it's length and that's not what you need on a painted surface or wood area.

The individual nibs look like old-fashioned phonograph needles, all welded together and the points are ground in one direction only.

That OTHER so-called nib file is a radially cut file - much like a single mill bastard, and is not the real thing although it too has it's applications.

202dy · #10 09-14-2010, 07:00 PM

Quote:

Originally Posted by SurferJoe46

No - look again.

The true nib file is just that - NIBS that are stood on end and they are ground all in the same direction to ONLY cut something that is standing proud and will NOT cut the surrounding material.

A FILE - OTOH - will cut anything along it's length and that's not what you need on a painted surface or wood area.

The individual nibs look like old-fashioned phonograph needles, all welded together and the points are ground in one direction only.

That OTHER so-called nib file is a radially cut file - much like a single mill bastard, and is not the real thing although it too has it's applications.

The Stew Mac file does not possess radial cuts. It is a single cut file. Like the nib file, the teeth share the severe forward angle and has safe sides. Both will perform the work admirably given the operator has the proper skills.

tofit m · #11 10-10-2010, 12:37 AM

I'm a wood finisher by trade and have never seen a nib file! Very cool tool! I use a comparatively shade-tree approach by putting a strip of masking tape down opposite sides of the defect and using it as a ramp or bridge to apply whatever filler I' m using. This causes the repair to be proud of the surface. Initially I sand the higher spots down, again using the tape as a ramp. When I get close, I take off the tape and feather out the repair. (The tape also minimizes the size of the repair area.) But the nib file is much, much cooler! thanks for that...