Fixing Cracks & Loose Neck Joints Caused by Humidity

One of the problems associated with buying an unfinished timber product, such as a DIY guitar kit is the difference in humidity levels between the country of origin and the region it is delivered to. In extreme cases, problems such as cracking, and/or changes to the physical properties of the body and neck can lead to problems occurring when assembling your new guitar.

Recommended: How do you change a nut on a guitar kit?

But fear not, in today’s article we’re going to look at how humidity affects guitars, particularly kit guitars and the steps you can take to deal with problems associated with humidity. So, if you have noticed your new kit guitar has developed a crack or are wondering why the neck pocket feels loose today’s article will help you address these issues.

What is humidity?

Before we get too far, let’s quickly discuss what humidity is. For the uninitiated absolute humidity is an indication of the amount of moisture in the air. Absolute humidity however isn’t a terribly useful marker because warmer air is capable of holding more moisture than colder air. For this reason, we measure humidity as relative humidity which indicates the percentage of moisture in the air compared to the maximum amount of moisture possible. So, for example, when we use the term 40% humidity, this means the air is holding 40% of the moisture it is capable of holding.

Why does humidity affect guitars?

Wood is hygroscopic, meaning the moisture content of the wood is subject to change based on the relative humidity. In simple terms, this means the cells of the timber can either expand or contract based on the amount of moisture in the air.

In the case of unfinished guitars, almost all problems associated with humidity will be due to the wood contracting due to reduced moisture content in the wood. This can result in both problems listed below:

• Cracking or a separation of the wood pieces the body is built from
• A loose-fitting neck pocket

Humidity also affects finished electric guitars, especially with regard to tuning stability and intonation, along with potentially raising the action due to changes to the neck profile. These are normally addressed by adjusting the individual saddles and truss rod. This kind of problem can affect touring musicians or those that live in regions with major changes in humidity through the year as a result of changing seasons. 

The most effective way to prevent this problem from occurring is to keep your guitar in a hard case and/or use some form of humidity control in the room your guitar is kept in, or the case itself. There are a number of products available for both purposes.

Right, now that we know what we are dealing with, let’s take a look at the most common problem associated with changes in humidity and the most visual, cracking.

Recommended: How to Finish A DIY Guitar Kit Neck and Fretboard

Repairing cracks or separation

All of the guitars at guitarkitworld.com (and almost every other guitar kit retailer for that matter) are manufactured in Weifang, Shandong, China which has a monsoon-influenced, four-season humid continental climate, with hot, humid summers, and cold but dry winters. It’s a humid environment as evidenced by the screenshot below showing the current temperature and humidity at the time of writing.

The humidity level in Weifang, Shandong, China

 If a guitar made in an environment that has a relative humidity of 65% is delivered to a desert region such as Las Vegas which experiences relative humidity levels as low as 8% the hygroscopic nature of wood means it is going to lose moisture. This causes the cells in the wood to contract, raising the potential for small cracks to appear. 

The humidity level in Las Vegas, NV, USA

This kind of damage is more likely to occur on the guitar's body, as the body is built from 2 to 3 different sections of wood, and despite being of the same species are likely to have different tensile strengths. These opposing forces can result in cracks occurring anywhere on the body of the guitar but speaking from experience they mostly occur toward the lower bout of the guitar. 

In most cases, the crack will be minor in nature, but in some cases, the crack will be much deeper. In either case, the steps below will help you address it.

1. Identify the problem

We first need to identify exactly what we are trying to repair. For example, is it a genuine crack? Or have the different sections of the body separated? Does the crack go right through the body of the guitar?

For example, if the crack extends right through the body we’ll need to address both the top and underside of the guitar, and the guitar will need to be clamped. 

2. Acquire the right products

When repairing cracks I recommend using CA (Cyanoacrylate Adhesive) glue, aka superglue. However, aim for a water-thin version such as this will more easily infiltrate the crack. 

You can also apply a tinted wood fill epoxy such as this if the crack is shallow and the guitar has reached equilibrium moisture content, meaning it matches the moisture content of the surrounding air.

You will also require a putty knife if using epoxy and sandpaper in both medium and light grades and a wood clamp or similar tool to fix the body of the guitar in place (If you don’t have a clamp don’t worry, we’ll cover this shortly) if the crack is deep.

3. Cleaning out the crack

The first step is to remove any dust or grit within the crack and surrounding area. A simple way to do this is to run a scalpel blade back and forth along the crack, removing any grit and residue and then wipe down with a very small amount of warm soapy water.

4. Fill the crack

If using epoxy, use a putty knife to work the epoxy into the crack. Remember in some cases epoxy comes unmixed, the hardener is separated to the middle of the roll. You will need to mix it around in your hands before applying so be sure to wear gloves.

If using CA glue fill the crack with sawdust from the guitar itself (if possible) as this will match the body. Identify any areas that will require sanding and use medium grade paper sand the area until you have a small amount of sawdust to work with.

Next, wearing a pair of gloves, begin to fill the crack with sawdust. Work the dust back and forth over the crack.

5. Add CA Glue

If working with water-thin CA Glue, using the supplied applicator apply glue into the crack covering the saw dust you have just applied. Ensure the crack is filled and then cover with an additional layer of sawdust and leave to dry. If you don’t have an applicator try blowing the glue into the crack through a straw, obviously taking great care not to inhale any fumes.

If the crack is the full length of the body e.g. the wood has separated or it is the full depth of the body use the same technique, however, be sure to clean and fill both sides of the body.

6. Clamping

If the crack is deep it should be secured using a clamp, this is especially the case if the wood has separated. Unfortunately, however, most of the time the area will be difficult to clamp due to the contours of the body or the width being too wide for the clamp to fit. 

In this case try using an occy strap, ratchet tie down strap, or similar item. Don’t over tighten, It’s a misconception that the clamp or alternative needs to be as tight as you can manage, the clamp’s main job is to keep the body stable and allow the glue to do its job.

7. Sanding

CA glue dries quickly, some fillers however take time to cure so be sure to check the manufacturer’s instructions before sanding. Once suitably dry, scrape and/or sand away the excess and inspect your work. 

💡Also read: Sanding a Guitar Body Prior to Painting

You may still notice a small line depending on the extent of the crack and the likeness of the fill material compared to the guitar's body, but the crack should be far less visible and if you are staining the guitar it will become even less visible as the material also absorbs the stain. Obviously, if you are using a solid color this isn’t an issue.

Repairing a Loose Neck Pocket

The second most common issue associated with humidity changes and kit guitars is a loose-fitting neck.

This problem also occurs due to changes in relative humidity, mainly as a result of the moisture content of the wood being reduced resulting in the wood cells contracting and causing the neck pocket to contract.

Aside from detracting from the look of the guitar this can also affect sustain. This is because without solid contact between the body and the neck vibrational energy is reduced.

The good news is, this is also a relatively easy fix.

Depending on the extent of the loose neck fit, the safest option is to use a veneer (aka shim) matching the wood the guitar body is built from or if unable to source matching timber using a hardwood veneer e.g. maple. 

I’ve listed the steps below, but keep in mind you shouldn’t use anything that will fix to the neck permanently e.g. using a filler if repairing a bolt-on neck as this will prevent you from being able to remove the neck in future if required. You can however permanently fix a shim to the edges of the neck pocket without any problems.

Fix a Loose Neck Pocket

1. Use a feeler gauge to measure the gap in the neck pocket
2. Order or cut two equal width shims from a matching timber, or maple (or a similar hardwood) and sand to an appropriate depth
3. Place the shim inside the neck pocket and using a pencil, trace the contours of the neck pocket onto the shim which will serve as a mark for cutting to size.
4. Cut to size, cutting just outside the edge of the lines to allow for the width of the blade and sanding once the shim is in place.
5. Glue the shims inside the edges of the neck pocket. 
6. Once the glue has cured begin to sand away any excess timber protruding out from the neck pocket.
7. Dry fit the neck and check your work. The neck should fit but should be a tight fit, meaning you need to apply just a small amount of pressure to insert the neck.
8. If you have over-corrected and the neck is now too tight, lightly sand both shims evenly and attempt to fit the neck again. Check your work regularly and don't remove too much material and risk having to redo the entire job.

You should now have a much tighter fitting neck ready for assembly.

💡Also read: Neck Assembly for Kit Guitars

If however, the gap is quite small, to begin with, failing a veneer you can also fit the neck and then apply a mixture of CA glue and sawdust or epoxy filler. Keep in mind however this is only recommended for set-neck guitars. Also, keep in mind if the gap is very small e.a a couple of millimeters, it may be non-existent once a finish coat is applied to the neck.

Summary

The information above should help you overcome two of the most common problems associated with excess humidity. Working with wood of any nature will present challenges from time to time, it’s part and parcel of working with organic materials. However, as seen in the information above, generally there is always a solution and in most cases, the solution is often simpler than anticipated.

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Marty Banner

With deep understanding of guitars, Marty Banner translates his expertise into engaging DIY guitar kit tutorials, helping aspiring and experienced guitar builders to handcraft their own musical instruments...

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