I have half a gallon of Titebond I (yellow PVA glue) that has thickened quite a bit, making it difficult to pour.
- Can I thin it? How?
- Will thinning it have any effect on the glue's strength?
According to Titebond you should first try repeatedly tapping the container on a hard surface to loosen it up. Then you can thin it with water up to 5% of the glue's weight or volume.
Regarding strength, they say:
Adding more than 5% water to our glues will decrease the bond strength.
This implies that thinning up to 5% will not decrease the strength of PVA glue. But, because it is simply restoring lost water, rehydrating the glue will not effect strength.
However, they also say that if the thinned glue is "stringy or pasty" and doesn't stick to wood, you should throw it out.
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The Titebond quote regarding thinning doesn't apply here as it is referring to dilution, from full-strength to a diluted form. – Graphus Apr 15 '15 at 10:05
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Can I thin it? How?
Yes. Add water.
Water is nearly all of what has been lost to the glue by evaporation (this is also how PVA-type glues set within a joint, by dehydration).
Will thinning it have any effect on the glue's strength?
No. Or at least no if you don't add too much.
You're not thinning in the conventional sense of reducing it from full strength to a diluted consistency, you're replacing lost water.
If you did happen to dilute too heavily all is not lost. The water hasn't in any way damaged the glue, it's just too dilute to be strong enough within a joint to perform properly. All you would then need to do is then let it dry out once more, e.g. by leaving the cap off for some hours.
There is a secondary issue however and this is any microbial life you may unintentionally introduce to the glue along with the water. You can get away with using tap water or bottled water, many people have used both and have not had a problem, but it's safer to use distilled water if you can get it.
