How To Thin Wood Glue

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Those of us who are proficient in the kitchen know that it’s important to put lids and covers on tightly to prevent food from going bad in some way.  Sometimes that bad can be a film forming on the top; sometimes, it can be milk going sour; sometimes, it can be mold growing.

Those of us who are also proficient in the woodworking shop know it’s important to put lids and covers on tightly, too.  Paint can lids, for instance, need to be placed tightly to prevent a skin forming or the paint separating, depending on what kind of paint it is.  Finishes like shellac, varnish, and polyurethane need to be re-covered tightly, too, for the same reason.  

Glues are no different.  Remember what the constituent parts of glues are, depending on the types of glues you have in your shop.  Water and chemicals can evaporate, and when they do, what’s left will usually harden to one degree or another.  Hardened glue might be beyond help, and you’re off to the hardware store to pick up a new container.

Lids are important, and replacing them on the containers needs to be tight.  It’s important, also, to remember that glues have a certain shelf life, and it is often smart to label glue containers prominently with the date of purchase to give you a head’s up when that shelf life is approaching.  It’s awful to have run out of glue just as we need it to join pieces of wood.

Types of Wood Glues in the Shop

We have three main types of wood glues to work with, and the use to which we will put the glue, as well as the place, will determine which type of glue to use.  

  • PVAPolyvinyl acetate, or what we woodworkers call wood glue.  The most well-known brand is Elmer’s Glue.  Most of us likely have a container of it in our shop.  Quick – check yours and make sure the lid has been replaced tightly, and then come back to finish reading this article.
  • Polyurethane.  Gorilla Glue is the best-known of the polyurethane glue brands.  It was the first company to introduce polyurethane glue to the United States market.  Want to know how strong Gorilla Glue is?  
  • Aliphatic.  The best-known brand of this type of wood glue is Titebond.  It’s a synthetic product, yellow in color and creamy in texture. 

Notwithstanding the fact that the original Gorilla Glue is the polyurethane type of glue, Gorilla Glue has also brought a PVA product to market – Gorilla Wood Glue.

Three different types of glue with recognizable brand names, and it’s not uncommon for woodworkers to have a container of all three types in their woodworking shop.  

Want to know why wood glue is so strong?  

What Causes Glue To Harden or Thicken?

If you’ve not replaced the cover or cap tightly on your container of any of these types of wood glues, there is a distinct possibility the glue has become thickened or developed lumps.  Improper storage is the leading cause of glue despoiling in some fashion or another.  The easiest way to keep glue from thickening is simply to pay attention to its storage. 

  • Heat.  Cool, dark storage is where you want glue to be.  Not cold, just cool.  Extreme heat will spoil glue and cause it to thicken or harden – water and other glue components evaporate in heat, so keep the glue cool.
  • Cold.  The key word is cool, not cold.  Freezing temperatures will spoil glue and cause it to thicken and harden.  So, again, cool, but not cold.
  • Shelf life.  We’ve mentioned this already, but it’s worth a second mention.  We mark our glue containers with the date of purchase and figure we have a couple of years of good use from it – as long as we put the cap or cover back on tightly.  
  • Exposure to air.  Again, as long as the cap or cover is put back on tightly, the glue will stay fresh absent heat and cold, and it’s still within its expected lifespan.

Milk spoils and gets lumpy if not stored properly within its shelf life; so does glue.  Be smart with both. 

How To Bring Back Hardened or Thickened Wood Glue

Well, in spite of our cautions here, you let the glue go, either by forgetting to replace the cover or cap or because of some other improper storage (heat and cold).  Now, what; are we off to the hardware store for a new supply, or can we bring it back to a useful consistency?

Bringing PVA Glue Back

This one’s easy.  PVA wood glues are water-soluble.  This is why they are so easy to clean up after – a damp cloth will wipe off excess glue from the workpiece and from your fingers. Water-soluble also means it can be thinned with water. 

While Sherlock Holmes chose a 7 percent solution, you want to stay at or under 5%.  You can thin a PVA wood glue with up to 5% of water against total weight.  Start slowly adding water, a few drops, and give the glue a stir.  Continue adding water up to the 5% until you reach the desired consistency.

We’re told that vinegar will also help thin a thickened wood glue, but we’ve yet to try this.  We follow our own advice and put the cap back on tightly.  We even squeeze excess air out of the container before doing so.

Bringing Back Polyurethane Glue

This one is a bit different.  Water will not rescue thickened Gorilla Glue or any other polyurethane glue.  Water will ruin the glue and make it a sticky mess that will be unusable.  That will result in a trip to the hardware store for you.

For polyurethane wood glue, try soaking the container in warm water; warm water, not boiling water.  Titebond’s literature recommends this as means of thinning out their polyurethane glue.  We suspect it will also work with Gorilla Glue and other polyurethane wood glues. 

We’re not sure about this next one, but our research suggests that some have found adding mineral spirits or paint thinner to polyurethane wood glues as another solution to thin it out for use again.  

Bringing Back Aliphatic Wood Glue

Titebond is probably the best-known of the aliphatic wood glue products.  As we mentioned earlier, Titebond literature suggests letting the container sit in warm water for a few minutes and give the container a good shake to test consistency.  If/When it reaches that consistency, you’re all set.  If it doesn’t get there, it’s time for a trip to the hardware store.

Can You Bring Back Thickened Epoxy Glue?

This is something of a trick question.  As you likely know, epoxy glue is mixed to order, with two parts being joined only in the amount you anticipate using.  As long as you’ve measured carefully, it will result in a glue of proper consistency.  But use it quickly.  

Once mixed, the epoxy glue has a shelf-life of between only 5 minutes and 60 minutes.  After that time, it solidifies and can’t be brought back. But, if it’s closing in on the expiration of usefulness and you need just a little bit more, we believe you can thin it with a little acetone.  That’s what’s used to clean it up, so we believe a drop or two will also thin it out long enough for you to finish working with it.  

What is Hide Glue and Can It Be Thinned?

Hide glue is made from animal hides.  You probably remember stories about horses being brought to the glue factory.  It actually was true in that some of the horse, usually the hooves, would be used in the making of glues.  

In the unlikely event you have hide glue in your woodworking shop, and after all, why would you when you can use the far more common PVA or polyurethane glues, and you need to thin it, use the same remedy as that for polyurethane wood glue – put the container in some warm water and give it a shake from time to time until it reaches the desired consistency.

As a final word about how to thin wood glue, we’ll repeat ourselves:  store it properly, and you’ll never have to thin it.  An ounce of prevention goes a long way toward that pound of cure.

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