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Frankg

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Reply with quote  #1 
I just read here in the forum about how not to glue painted surfaces. I have done it and it seems to have held. Are there ultimately consequences for doing this? Like, it will lose it's bond over time?
How about gluing a painted surface to a bare wood surface? Does the same apply?
Due to this fear [eek] I have sanded the areas where the paint was applied, then glued. Other times, due to laziness, [sneaky] I have also glued painted surfaces together. [frown]

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cynthia lewman

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Reply with quote  #2 
Hi Frank,

The concern with gluing painted or stained surfaces is the glued joint will not be as strong as gluing bare wood. Paint and stain block the pores of the wood which prevent the glue from penetrating the wood. A joint that is glued and clamped between two pieces of bare wood is usually stronger than the wood itself.

The true test of gluing painted or stained surfaces is when the toy is played with and pressure is applied to the glued joint. With light play the joint might hold OK. We've received a couple of emails over the years from customers who said the riding toy they've built from our plans broke after it was played with. We then discovered that these customers glued painted parts which ultimately caused the joint to fail. When the toy was rebuilt gluing and clamping bare wood surfaces together, the joint held perfectly even after weight and pressure was applied.

Here's a quote off of Titebond's website:
Most of our glues are designed to bond bare wood. Painting or staining a wood blocks the pores, keeping the glue from penetrating into the wood. The Titebond Polyurethane Glue may work for gluing together painted or stained surfaces, but it is necessary to remember that the overall bond will only be as strong as the bond between the paint and the wood. We recommend that all substrates be clean of any type of paint, stain, or sealer.

Titebond has an excellent FAQs page about gluing wood including how to disassemble a glued joint. 
Udie

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Reply with quote  #3 
Cynthia - Great reply post on gluing concerns with respect to painted/stained surfaces. You are the best.

Frankg - My personal preference is to always try to glue to unpainted surfaces when possible as demonstrated in the May 23, 2014 Freaky Ford Coupe WTN and video. However, this as you know is always not possible. One of my solutions, which has been successful, is to score the mounting surface with either a X-acto blade, utility knife and some times a chisel. By scoring the surface I am removing some paint and at the same time making grooves in the wood, exposing an unpainted wood surface.
When the mating component is glued on, the glue will go into the grooves and also cover the back surface of the mating component. When the glue cures the glue filled grooves act as ribs aiding anchoring and gripping, hence providing more support for any up/down and sideways pressure. On occasions I have also made grooves in the mating component also.
I have seen other woodworkers, drill a series of shallow holes, creating recesses for the glue to pool into, again providing pockets for anchoring the mating component.
Another solution, which really is dependent on the size and shape of your mating component is to drill a hole and insert a small dowel or even some round toothpicks as anchors. The holes in the mating component do not have to be the same diameter or require precise drilling. Make those holes larger, if possible,  allows you to  have the ability to freely position the mating components to the main assembly. You can also reverse this technique by putting the pins in the mating component and drill the larger holes in the main assembly if that is more convenient.
Hope this helps.
Ken Martin

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Reply with quote  #4 
Hi Frankg

When I forget, which is more often than I can remember, and paint parts that are supposed to be bare wood glue joints, I like to use Udies scoring method with a sharp blade.
Sometimes if I have enough room on the piece I will sand away the center of the painted area beck down to bear wood.  If the primer or paint is fairly fresh this works really good.

Cynthia's response is the best answer, as it is better to do it right the first time.
However, it seems forgetfulness happens more often as time passes for me.

Oh, what was the subject again?

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Frankg

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Reply with quote  #5 
Hi All,

I never thought of it that glue will not penetrate the wood pores if it is covered with paint. That is a real lightning bolt for me. Makes sense.
It is best to understand the reasons why some "do not's" are stated. Otherwise we are likely to ignore them.

Thanks Cynthia for this great fact of gluing.

Also I want to thank Udie and Ken for their excellent tips on how to overcome the "Forgetfulness" problem!! Great tips!!

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Frank Galica
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