Acid Brushes: Small but Important Details

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

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

About Lost Art Press

Publisher of woodworking books and videos specializing in hand tool techniques.
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26 Responses to Acid Brushes: Small but Important Details

  1. wb8nbs says:

    Another hot tip:
    An old 3 1/2″ floppy disk makes a good glue spreader for large flat surfaces.

  2. peter j marshall says:

    Thanks for the timely article and I will start trimming those floppy acid brushes ! On a related subject , what is your preferred glue ? I have switched over to the Titebond Hide glue from PVA after reading one of you past articles and it certainly helped when assembling dovetails and for cleanup of squeeze out . Any other guidance / suggestions on what you are currently using ? Is it worth going the extra step to using a hot hide glue ?

    • I use Old Brown and Titebond liquid hide for most joinery. I use Titebond I for panel glue-ups. We do use hot hide on occasion, but I generally prefer the liquid hide.

      • Rachael Boyd says:

        one small point, liquid hide has a shelf life. it has caused me stress in the past so I always check.

        • peter j marshall says:

          Rachel , I checked the code on a bottle of liquid hide glue that I purchased today and it was manufactured in 2017 . I will be returning this tomorrow ! . Thanks for the heads up ..

  3. T Wright says:

    i use tuna cans for glue. impossible to accidentally knock over. they can still be pushed off the edge of the bench though.

  4. pinusmuricata says:

    I keep a few of these around, but use them rarely, mostly for throwaways, like for epoxy, barge cement, etc, but don’t care for the tendency to shed. Over years I’ve collected a bunch of cheap artist brushes in a variety of sizes and shapes that I use for glue spreading. They go into a jar of water as soon as I’m done spreading, later washed. For spreading over a large area, like veneer, I use a cheap plastic spreader with notched edges sold for spreading tile mastic, etc.

  5. Jonathan S. says:

    A tip I picked up from Tommy MacDonald. Before trimming the acid brush, give the ferrule a whack or two with a hammer. It helps to hold onto the bristles better.

  6. Jonathan Sszczepanski says:

    A tip I picked up from Tommy MacDonald. Before trimming the acid brush, give the ferrule a whack or two with a hammer. It helps to hold onto the bristles better.

  7. Mark Kessler says:

    I like to put a little super glue at the ferrule helps with losing bristles and o always have trimmed the as described

  8. jonathanszczepanski says:

    A tip I picked up from Tommy MacDonald. Before trimming the acid brush, give the ferrule a whack or two with a hammer. It helps to hold onto the bristles better.

  9. Rachael Boyd says:

    I do the same thing but I don’t like acid brushes ( its the metal handle I don’t like). so I pick up a hand full of 1/2″ wooden handle throw a ways, trim them up the same as you do. I like that if you forget to clean them and find one a couple days later put it in a can with a little water and for a day and they are good as new. I have one in my hot hide glue pot also but I trim the bristles a little longer so it can hold more glue. they last for years.

  10. Steve P says:

    You know I recently learned this trick when building a Ukelele kit from StewMac. Their build videos recommend getting these acid brushes and cutting the tip into a shorter point to glue details and also use with water to clean glue squeeze out, works especially good in tight corners.

  11. Ron Michaelsen says:

    The toothbrush trick is genius!

  12. Kevin Almeyda says:

    And though acid brushes are pretty cheap at The Home Depot, they are REALLY cheap on Amazon. Also, while silicone brushes sound like a good idea, I feel they’re not in actual use. Too wide and don’t hold enough glue.

  13. I do the same, and also use Old Brown glue for joinery, and Titebond I for panels and jigs. Never use Titebond II. It’s both too waterproof, and not waterproof enough when you need it to be. Glue brushes will become throwaways with Titebond II, and if you’re used to wiping glue on your tee shirt, as I do, II won’t come out in the wash, where Titebond I and hide glue will.

  14. Jacque Wells says:

    As much as I hate common sense (especially when predicated on experience), you are to commended. I think I have the ‘nut’ of the lesson. Thank you. Excuse me, I gotta go buy some brushes.

  15. Robert Hobbs says:

    Thanks for sharing great tips some others might keep secret!

  16. David says:

    I use the acid brushes for epoxy and poly glue and then toss them. I sometimes trim them but I find I always have to tighten the clamp on the bristles with pliers or a lot of them fall out in the joint. For wood glue I use acrylic artists brushes from Joann Fabric or a discount store. Available in many sizes and shapes, don’t absorb the water in the glue, and the bristles stay put. Even with consistent hot water washing, though, I find they only last maybe six months before they become misshapen and lose their resiliency and shape.

  17. David says:

    Oh, by the way, the toothbrush idea is great. I use toothbrushes for other shop tasks, but that never occurred to me, thanks!

  18. Jeremy says:

    For small brushing needs, I’ve started using bamboo skewers that I tap quickly with a hammer/anvil to make into a stiff bristle brush (more flexy if you tap a longer length) if you are truly cheap, you can snip the end and repeat a long time.

  19. Fred says:

    Previous comments are great and a flux brush is an additional purchase while a toothbrush to spread PVA glue is typically not. A retired toothbrush will spread and force water soluble glue into wood fibres over smaller areas without exquisitely fine details on both sides of a joint without the need for another purchase and will clean well after an hour or two of simply soaking in a water bath without loss of shape. Using the durable bristles of a toothbrush to spread glue over a larger area is slower and may be a fool’s errand. The manufacturer of the toothbrush took care of keeping the bristles in place, though the form factor of the package does rule out the toothbrush as a tool for applying glue to all but the entry bits of a mortise.

  20. Another tip is to spread glue all over your nose and let it dry. Once dry, peel it off and remove all off your blackheads.

  21. Francis Coppage says:

    WOW. This made my day! I’ve only been doing wood working seriously for about 12 years (since I retired), but I’ve used flux brushes for glue-ups from the git-go. AND right out of the gate I was trimming them down to about +/–3/8″. I started using the flux brushes because a friend of mine as a plumber used them for there intended use and bought them by the gross. He suggested the use for glue ups, but. as you pointed out, just too floppy for any precision. Maybe there is hope for me yet!

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