Author Topic: [need some help] Paint butt joint (so you don't see the seam)?  (Read 2244 times)

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Offline threesixright

  • Posts: 643
Hi All,

I hope you had all great x-mas days!

I want to butt join (using domino) and then paint two pieces of plywood (12mm).

I can imagine if I just glue then together, sand and paint the boards, you will always see the "seam".  Anyway to glue it together without the seam being visible?

I was thinking to take of the corner with a router or a plane on both sides, so when you connect the boards you get a v-groove. The v-groove I would fill with some wood filler, than sand and paint. But not sure if this would be the best way of doing so? Wood will also place outside  [blink].

The groove would be something similar to my sketch below.

Appreciate your thoughts. Thanks!



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Offline Doug S

  • Posts: 537
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #1 on: December 27, 2019, 07:04 AM »
Not sure if it is the same elsewhere but here in the UK that is how some firms make their windows etc.

The theory is that for example on the sashes where the stiles are jointed to the rails you can get some movement causing the paint to crack which can let water in.

To overcome this they take a small bevel off as you suggest and fill the joint with a flexible filler, this allows for a small amount of movement without the paint failing on the joint, that is the theory anyway!

Offline Lincoln

  • Posts: 138
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #2 on: December 27, 2019, 07:07 AM »
Can you tounge & groove the join, instead of domino? Or, leave the 'V' as it is and paint it without filling?

Offline Cheese

  • Posts: 8759
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #3 on: December 27, 2019, 10:15 AM »
The problem I see with the filler is that if there is any movement in the materials, instead of seeing a single witness line, you'll probably see 2 witness lines.

Can you make a small v-groove at the joint (using a small veining bit) that will serve as a decorative touch and still hide any potential joint movement?





Offline Alex

  • Posts: 7570
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #4 on: December 27, 2019, 10:48 AM »
To overcome this they take a small bevel off as you suggest and fill the joint with a flexible filler, this allows for a small amount of movement without the paint failing on the joint, that is the theory anyway!

That's indeed the theory, in practice it is incredibly difficult because the wood keeps moving and even the flexible filler is not flexible enough to keep it together forever.

Also, the problem with a flexible wood filler is that it will not be 100% flat like an inflexible wood filler will be.

Best thing is too avoid these seams at all if you want a completely flat surface.

Offline Michael Kellough

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Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #5 on: December 27, 2019, 11:03 AM »
I agree with Alex, but “forever” is a long time. A flexible filler (some kind of Sikiflex?) will keep water out of the joint for a long time. Much longer than an inflexible filler. Seal the edges of the boards after chamfering and before joining. Oil based primer Alex? The filler/caulk will stick better to the sealer than to raw wood.

Another option is to use a scarf joint instead of a straight butt joint. Doesn’t need to be a long scarf, not even 45 degrees, to get gravity assisting in removing water.

Echoing Alex, best to change the decision to avoid the horizontal joint.

Hmm, I was just assuming this would be outside.  [huh]
« Last Edit: December 27, 2019, 11:08 AM by Michael Kellough »

Offline Alex

  • Posts: 7570
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #6 on: December 27, 2019, 01:41 PM »
Oil based primer Alex? The filler/caulk will stick better to the sealer than to raw wood.

Makes no difference if you use oil or water based primers. No inflexible filler sticks so good it can overcome the movement of the wood, no matter how you treat it. Two joined pieces of wood just move too much, day in, day out and you'll see cracks appear eventually, sometimes after 3 months, sometimes only after two weeks.

Inflexible filler is only good inside a single piece of wood, but will eventually crack too, but now it might take a couple of years instead, in line with the natural re-paint cyclus anyway.

Flexible filler is what you use in places where two pieces of wood join. But the nature of flexible filler is that it will keep on moving so it wil not come loose and cause cracks, but due to its flexible nature it will not ever be 100% flat like inflexible filler. You can sand it flat and paint it and it will stay like that some time, but after a few months you'll see it is slightly bubbled, sticks out, or is recessed. Not much, but enough to see. You don't use flexible filler right in the middle of a surface if you want it to stay as flat as a mirror.

If you want a seamless surface, outside no less, you'll have to step away from wood and go for something that is plastered.

Offline FestitaMakool

  • Posts: 1010
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #7 on: December 27, 2019, 05:19 PM »
Joining sheet goods, calls for: I think it says in English; stile & rail.
A lot of sheet goods here comes with these profiles as standard for joining.
These are meant to be glued. These joints won’t move when glued, the force would be extreme to let go. If you could do this with a router instead of dominos, I think it’s safe to say that you can fill (preferably a “V” grove, albeit narrow) with acrylic based filler. These fillers allows for slight movement, if the paint does..  [huh]
“The Stig” Yes, it is true, at least some part of it..
“If you have an old Land Rover and a fit wife, you’re most likely always busy”

Offline threesixright

  • Posts: 643
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #8 on: January 03, 2020, 10:59 AM »
Hi All,

Thanks for all the replies! Noob as I'm, I should have mentioned a few things.

- Its outside (basically the boards that go at the roof edge)
- My first concern was the water getting into the joins
- My second concern was the aesthetic side of it (dropping the wish to make it one seamless surface)
- Measurements (FWIW) ~ 10 x 15 x 10 x 15 (rectangle roof, in meters).

I'm rethinking the joints (thanks for the ideas) and think to make something like a small visible groove.


Offline jobsworth

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Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #9 on: January 03, 2020, 11:31 AM »
Bondo or Mohawks filler.

I just discovered it and like it.

Since this is outside maybe some caulk would work cuz ya need something flexible for the wood movement


Offline Gregor

  • Posts: 1752
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #10 on: January 03, 2020, 08:20 PM »
- Its outside (basically the boards that go at the roof edge)
I would determine minimum view distance of the seams and do a real world test through inspecting sample seams at that distance. You might feel more relaxed afterwards, especially should you factor in that normal people (= everyone except you who installed the thing) will likely never, at all, notice that what you worry about.

In case you want to glue the seams I would suggest to use a #491034 or similar and a good waterproof adhesive. In case we're talking fascia boards a better option could be to use non-permanent fasteners (eg. domino connectors) so you can access/replace individual parts in need of service without having to tear everything apart completely/destructively.

Offline Scott Burt

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Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #11 on: January 03, 2020, 08:27 PM »
I think this one is being over thought.

Glue the pieces up...do dominoes or biscuits or dowels or whatever, or just glue...

Sand.

Oil prime.

Lightly sand smooth.

Paint.

Done.

Anyone says the can see the joint, show them the door.

Offline Alex

  • Posts: 7570
Re: [need some help] Paint butt joint (so you don't see the seam)?
« Reply #12 on: January 04, 2020, 12:46 AM »
- Its outside (basically the boards that go at the roof edge)

These things (Boeidelen) are always placed with seams visible. Nobody is ever bothered by them. Just make sure you seal the edges before installing them. Mostly that is done with a few layers of paint primer, thinned, so it can penetrate the wood very well. You can also do it with wood glue as Scott suggested. However, the boards are never glued or dominoed together, only fixed directly to the roof. Make sure you use high quality outdoor wood panels like okoume or red cedar, or else you can get rot anyway, sooner or later.