How To Stop a Crack in Wood From Spreading

How To Stop a Crack in Wood From Spreading

Is a crack in a piece of furniture, or a floorboard, a stair, or a table fatal?  Does it mean you have to replace and rebuild?  Or is there some much less drastic measure you can take to cure and stop the crack from spreading?

Relax and take a long, slow, deep breath.  Of course, there are steps you can take to both fill the crack and stop it from spreading.  This is woodworking, and the limit of possibilities is only with your imagination, and a few tips can expand that for you easily.

We’ve even written of remedies in the past when we answered the question about filling cracks in wood.  You’ll find that previous piece here.  In that piece were discussed filling large gaps and cracks in wood, and the options haven’t really changed that much.

We’ll recycle some of that information and discuss the options, but this time with an eye toward treating the cracks in such a way that they don’t spread.  We’ll identify the “fillers” for those cracks, as well as how to fill them so they last.

Why Does Wood Crack?

The answer is shrinkage.  Remember that Seinfeld episode when George Costanza was afflicted with “shrinkage” after swimming in cold water?  It’s not exactly that kind of shrinkage, but sort of.

Wood will experience splits and cracks, known as wood checks in woodworking terms, because wood shrinks as it dries.  Wood dries twice as fast along the growth rings as it does across them, and it is this uneven shrinkage that causes splits and cracks.  

Weathered Wood Cracking

It is natural and inherent in wood that this will happen, and unless we encase wood in some polymer forever, like the princess encased in ice or glass, that shrinkage will occur.  You can count on it, and as long as you have a good understanding of filling those cracks, you’ll be fine.  A little effort, well-applied, and the problem is solved.

Filling Cracks in Wood

When it comes to filling those splits and cracks, you have a few choices.  Some of them are store-bought, and one is only half store-bought.  They include:

Water-based wood fillers.  This is crumbly in its natural form but is smoothed out with a little water.  It is easily applied, cleans up quickly, and dries fast.  A little sanding to smooth it flush with the surface, and the crack is filled and ready for a matching finish.

Gorilla All Purpose Wood Filler, 16 Ounce Tub

Petroleum-based wood fillers.  Comes already smoothed, needing only to be applied to fill the crack.  Because it is petroleum-based, it stands up better to moisture.  Remember that the crack is caused by shrinkage during drying over time, so keeping moisture out prevents the need to dry.

Wood putty.  This, too, is already smooth and very flexible.  And like petroleum-based wood fillers, it resists moisture and temperature fluctuations.  Because of its composition, though, it can be harmful to raw wood.

Epoxy fillers.  These come in both putty and liquid forms and dry for sanding in as little as 15 minutes depending on the volume used.  It will fill holes and cracks well and is a good choice for woods that have a little decay in them.  It has become a popular product to use on tabletops for aesthetic reasons, too, as color can be added to epoxy to add a new touch to the surface.

Powdered Fillers.  A little water, a good stir, and a paste is created from the sawdust and wood shavings in the powder.  This is good, though, only for minor splits, rather than cracks and large gaps or holes because it is not structurally firm.

Homemade fillers.  Made with items you likely already have in your woodworking shop, this is an inexpensive and handy option and in certain circumstances can be an effective choice. We’ll get to that in a little bit.

Using a Wood Filler or Epoxy to Fill Cracks

Epoxy Resin

Because wood fillers and epoxies come in liquid form, they are the smart choice for cracks that are deep, thin and long.  The liquid will seep deeply into the crack and extend along its run, reaching all exposed surfaces in the crack.  But, there are differences.

  • Wood filler.  A good choice for surface work where the cracks are wider or there are chunks in the wood missing.  Wood filler can be sanded after drying fully and painted to match the rest of the workpiece – a piece of furniture, a stair, etc.  
  • Epoxy filler.  This, on the other hand, is the better choice when the cracks are thinner and deeper.  It’s thinner than wood filler and will sink deeper into the crack, penetrating well into all of the little crevices of wood fiber in the crack.  It is also more structurally sound than wood filler and will form a stronger bond with the wood surface in the crack.  It dries harder and is more long-lasting than wood filler, too.

After you’ve made your choice of filler, gear up.  Wood fillers are benign, but epoxies can be poisonous and noxious, so a respirator is advised if you are working indoors.  You don’t want to heave all over the workpiece in the middle of the task.

Then, just follow these steps:

Be dry and clean.  Make sure the workpiece is fully dry and clean of debris.

Knife it.  If using a non-liquid wood filler, use a putty knife to push the filler into the crack.  Make sure to apply as much pressure as is necessary to reach its full depth.  You’ll know when the crack can take no more. Don’t obsess about smoothing it fully, as fillers can be sanded smooth after it has fully dried.

Pour it.  If you are using a liquid wood filler, give it a good stir and pour it into the crack.  Make sure you pour enough to fill it completely.  Again, it can be sanded, so don’t fret about it being smooth right away.

If using a liquid epoxy, simply pour it all along the crack until you are sure it is full and the epoxy has penetrated the full depth of it.  Remember that epoxy comes in two parts – the resin and the hardener.  Be sure to mix enough to fill the crack completely with a single pour.  This would also be the time to add a color if you have decided to spruce up the workpiece a little for aesthetic reasons.

Dry time.  Each product will identify the dry time necessary before continuing to work with your filler of choice.  Follow the manufacturer’s recommendations.  Epoxy will dry faster, too.  Each can be sanded smooth, and the wood filler can be painted or stained when fully dry.

Homemade Wood Fillers for Cracks 

Sawdust

This is an easy recipe to follow.  Every woodworking shop will likely have some Elmer’s glue handy.  And, because it’s a woodworking shop, it will likely have some sawdust.  It is especially helpful to have sawdust from the wood that comprises the workpiece to match up color and appearance.  

Mix them together into a paste and apply to the crack.  Again, non-liquid wood fillers are better for shallow and wider cracks in wood, and the home brew will be applied with a putty knife.  

Another option is to fill the crack with glue and then press sawdust into the surface.  This is a less pure way of getting the job done, and sanding might reveal the glue beneath.  So, go the extra step and mix the two and push the paste into the crack with the putty knife.

Allow ample time for drying, at least 24 hours, and then sand and paint or stain to match the workpiece finish.

A final thought on filling cracks has to do with prolonging the need to do so by adequately sealing the wood after it has fully dried so that moisture does not enter it.  Again, it’s the shrinkage that causes the cracks, and sealing that is moisture-tight can prevent cracks from developing.

A good oil will do that for you.  The most commonly used for this purpose include Danish Oil, Teak Oil, Tung oil, and to a lesser extent, linseed oil.  They also allow the beauty of the wood to show.

And, of course, shellac will provide the same degree of protection from moisture.  We’ve written of this in a past piece which you will find here.  There is also the protection provided by polyurethane, a film finish, that coats wood in plastic and prevents moisture and water from entering the wood.

But when you do encounter a crack in wood that needs your attention, you now have some tips on the best approach to filling it.  It’s not the end of the world and is an easy fix.