Acid Brushes: Small but Important Details


We use acid brushes to apply glue in our shop, but we tune up the brushes before using them.

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Straight from the store, the bristles are too long and wide. When they get wet with glue, they act like a floppy mop and make it difficult to apply glue where you want it and in the right amounts.

To tune up your brush, grab a sharp pair of scissors. First trim the bristles so they are 3/8” long. Then trim the width of the bristles. Basically, you want to make the bristles 3/8” long and 3/8” wide – square, some call it. If the bristles are too wide, you’ll have trouble getting into mortises without splashing some glue on the rim.

After glue-up, clean the brush (some of our have lasted five years or more). Check for any loose bristles and trim them back.

A proper glue brush is just one of the rituals in our shop. A few others:

  1. When assembling joinery, we rarely use glue straight from the bottle. We pour what we need into a paper cup (or coffee mug). The cup allows us to brush on glue or, in some cases, pour it onto a large surface if necessary.
  2. When we clean up squeeze-out, we use a toothbrush wetted with clean, warm water. The toothbrush gets into corners no rag can manage.
  3. We let things dry overnight if possible. You might be able to take the clamps off in 30 minutes. But if you don’t have to, why not leave the assembly in clamps overnight?

— Christopher Schwarz


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