PMF question...ever had this happen?

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PMF question...ever had this happen?

Postby Patrio » Tue Apr 07, 2020 12:25 pm

Fabric over Titebond II over 3/8" plywood.

First coat, everything went fine.

Second coat, this happened.

Image

What the...!

(The crooked upper line is a roller mark, ignore that.)

I thought, "Why is that stuff bubbling?" I rolled the roller over it a few times, and it wouldn't go down. The fabric is NOT bubbling. The plywood is swelling underneath. Everywhere else on this panel is perfect, but this just stands out.

Now, functionally, this is not a big deal because the fabric is in firm contact with the wood, but is there a chance this will "contract" as the paint dries? What causes this to happen, and how do I avoid it in the future? Is there a quick and easy fix, short of pulling the fabric and hitting it with a belt sander?

Maybe mash it down with a claw hammer? LOL.

Thanks in advance.
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Re: PMF question...ever had this happen?

Postby Pmullen503 » Tue Apr 07, 2020 2:50 pm

Bummer! In the future, Paint your plywood with the diluted TB and let it dry. If it checks again you'll have a chance to fix it.

That must have been some cheap and/or interior grade plywood.

You might be able to push it back down with heat. Let it dry completely and then place a cloth over the area and iron it with a lot of pressure. You'll need something like a block of wood to press it down after you remove the iron to hold it until it cools. You need to heat long enough to melt the glue under that top veneer and hold it till it cools back down.

If that doesn't work you can try injecting glue and clamping it down until the glue sets.
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Re: PMF question...ever had this happen?

Postby John61CT » Tue Apr 07, 2020 3:40 pm

If the actual plywood is deformed causing that ridge, have to either live with it or start over, IMO with a new piece.

Fully sealed marine grade, strong consistent and and "waterproof".

Otherwise you've got to seal it yourself first, and there's a risk level, compatibility issue, adhesive actually sticking to that added layer rather than the "wood" directly.

You definitely do not want a reason to skimp on the quantity of adhesive, need to saturate the fabric thoroughly from the "back" not just the front afterwards.
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Re: PMF question...ever had this happen?

Postby Patrio » Tue Apr 07, 2020 4:23 pm

Definitely too far in to re-do it at this point.

That's a 2x2 frame with Lauan on the inside and plywood on the outside, with insulating foam in between, with the panels attached with Liquid Nails and brads from my brad nailer.

I'll probably just live with it if I can't heat it and weight it down flat with something. I don't think it will cause any structural problems, but I will always notice it!
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Re: PMF question...ever had this happen?

Postby GPW » Wed Apr 08, 2020 4:43 am

Just saying , a Snappy Paint job covers many sins … ;)
There’s no place like Foam !
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Re: PMF question...ever had this happen?

Postby swoody126 » Wed Apr 08, 2020 7:51 am

this is a common issue with lesser quality plywood

when the log is rotary cut/peeled to create thin plies w/o loosing any wood to saw kerfs as in cutting with a saw the unrolled sheet gets really wonky

lesser quality plywood manufacturers utilize fewer plies and poorer quality glues for a given thickness

depending on which side of the peeled sheet was placed out the result is simply what you are seeing/experiencing

when building little what i call "crash boats" for kids to learn in i too have used the lesser grade of plywood primarily due to the time it takes me to drive 6hrs each way to Houston to procure the good stuff

i learned from an old NA who is a TiteBond II proponent of thoroughly coating the finished hull w/ slightly thinned TB II until the wood will accept no more in the beginning of the finishing process which helps(doesn't completely solve) w/ this issue

we finish w/ gloss latex(water based) trim paint which after being allowed to cure completely provides a really good finish

using this method i "hot coat" through out the entire process keeping each coat from curing which then requires sanding between coats

i realize it is too late in this case butt there will be others reading this thread who may benefit from seeing what can happen when using lesser quality materials

i am not saying that buying/using the good stuff will absolutely solve this problem butt the better grades of plywood have far less incidents like this one

for projects i build w/ the intention of keeping them as long as reasonably possible my goto plywood is what's called BS1088 Marine Plywood

https://www.midwestboatappeal.com/marin ... r/bs-1088/

IMHO TearDrop n Tiny Travel Trailers are not unlike boats in that they are subject to extreme weather and water exposures and the best available construction materials will provide greater longevity while maintaining their beauty for the duration

if you take the time to read the above link you will see that the 1/4" good stuff is 5-ply and made up of whole sheets w/ no voids

my personal experience is that even the edges of these products can be bright finished beautifully

fullsizeoutput_f79.jpeg
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just an old man's 2¢ worth on this already warm morning in Texas

sw
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Re: PMF question...ever had this happen?

Postby tony.latham » Wed Apr 08, 2020 9:13 am

The plywood is swelling underneath.


Ouch. What kind of plywood is this? 3/8" CDX? AC maybe?

:thinking:

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Re: PMF question...ever had this happen?

Postby MickinOz » Wed Apr 08, 2020 12:52 pm

If it has just swelled, it may sit back down as it dries if you are lucky. In the first instance, that is what I would do - wait for it to fully dry.

If it has actually delaminated due to not having waterproof glue between the veneers, I doubt it will re-glue itself.
You could certainly try maybe heat and weight, but I believe the PMF method involves saturating the timber with Titebond?
I doubt anything you could do will make a difference now.

A veneer saturated with cross linked PVA is going to be strong.
So to me it is an appearance issue not structural.

In any case, although I've never used it, that PMF is pretty tough, it should hold it all together regardless.
After it has all dried and settled into its final shape you could sand it under flat and patch and feather in a new bit of cloth, maybe.




What swoody126 said, with a comment or two.
Over here marine ply is expensive, more than double the cost of lesser plywoods.
Given the time and other materials that go into a teardrop, that shouldn't be an issue.
Except, the only size we are likely to get is 8 x 4 (British Standard 1088) or 2400 x 1200 (Australian New Zealand Stardard 2272), and as I build my 5 wide x 9 long, there is a lot of wastage.
I have never seen the 5 x 5 plywood that seems to be available elsewhere on the planet.

The things they have in common - they must be made with an A Bond glue line (fully water proof), and must have no interior voids.

I ended up compromising.
We get a structural ply product that's complies with a different standard.
It's pretty rough appearance wise, unlike Marine Grade ply it doesn't have an "A" face. Luckily, I have found if I tell the dealer what I am using it for, they will happily help me pick through the pack to find the best looking sheets.
And because I am cutting everything to 5 wide, I can usually cut out the worst of the surface defects.
It has a strength rating so can be used as a structural element, but most importantly it has the A-Bond waterproof glue.
I successfully bent a 7mm (a bit thicker than 1/4 inch) piece around a 23.5 inch radius by pouring boiling water on it.
No de-lamination at all, though there are some surface splits to fill.

The manufacturer's rep said to me that the structural ply is in his opinion good enough for the job.
I had told him why I wanted the waterproof glue and that I was going to seal it with multiple coats of diluted marine poly before painting.
He said "mate, I'd be telling you that marine ply needs sealing to make it weather proof anyway."

Having said all that, if I built again, I'd make the trailer 8 feet long, and use the marine ply for at least the side walls.
Why - because if I get the BS1088, it will be a full 8 feet long, and as straight as a straight thing.
I was looking at some the other day that was laser straight.
Much more stable than structural ply.
I am spending a lot of time eyeballing my build and being careful to pull everything straight as I join bits of wood together.
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Re: PMF question...ever had this happen?

Postby John61CT » Wed Apr 08, 2020 2:03 pm

MickinOz wrote:I believe the PMF method involves saturating the timber with Titebond?

PMF for Foamies, wood is not necessarily involved much.

And IMO better to skip the TB2, as long as the primer / waterproofing sealant that saturates the fabric is also a stronger adhesive, all in one like the original Glidden Gripper used to be.

But since that is gone, a replacement has not yet been put through the required testing regime by the community.

> marine ply needs sealing to make it weather proof anyway

Yes fully encapsulating even before assembly would be ideal.

And then again when skinning.

Personally I like the idea of taking wood out of the picture completely, would prefer the core structural to be materials that never rot, e.g. aluminum + insulation foam.
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Re: PMF question...ever had this happen?

Postby MickinOz » Wed Apr 08, 2020 4:15 pm

This chap has used Titebond on ply, though, so I doubt it is going to move.
I did wonder about that - I thought PMF on timber was more usually adhered with paint.
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Re: PMF question...ever had this happen?

Postby MatBirch » Thu Apr 09, 2020 12:11 am

I used titebond 2 for my PMF over Luaun ply. I had a similar problem in a couple spots but they are very small. For yours, I would try to find a syringe and inject a little TB2 into it. Let it dry for a couple of days, then hit it with an iron. It will melt the glue,and you’ll be able to press it back flat. Piece of parchment paper will keep your iron clean and won’t affect adhesion of the next coat.

Or... you can trim it out with some strategically placed twill tape as trim

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Re: PMF question...ever had this happen?

Postby Patrio » Thu Apr 09, 2020 5:08 pm

Well, I got mad and rageripped the canvas off.

There was a slight crack that allowed moisture into it and caused the swelling.

I filled it with wood filler, sanded it smooth, coated it with glue, then re-covered it.

I've done one coat of paint and it looks pretty good. I'll post another pic when I get it done.

Yes, this plywood could be higher quality...I'm definitely on a tight budget. I think it'll be OK though.

Thanks for all the info!
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Re: PMF question...ever had this happen?

Postby GPW » Fri Apr 10, 2020 4:07 am

Pat , Good fix !!! :thumbsup:
There’s no place like Foam !
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Re: PMF question...ever had this happen?

Postby Patrio » Fri Apr 10, 2020 10:17 am

Just finished the third coat. Looking good!

Image

This pic "hides" it a bit. It's still visible, but not noticeable at all. I'm happy with it.

Now, the second question...can painted canvas that's otherwise undamaged be re-glued to another panel? Because I'm going to try that next...lol.



It ripped in a few places near the edge, but otherwise, it came of in one piece. It feels like a big, heavy, stiff canvas tarp.

I'm thinking of trying to re-use it on another panel. If I can get it to lay flat, and glue it down well, and hit it with one more coat of paint, I'd think that would work.

In fact, why couldn't you paint your canvas first and cut it into sections, and then glue it on like siding, with the laps facing down? Sure, youd have seams, but then again, aluminum skin has seams...

Not gonna try THAT, but it's interesting food for thought when dealing with compound curves and the like.

I'm gonna experiment a little, lol. I'll post back.
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Re: PMF question...ever had this happen?

Postby John61CT » Fri Apr 10, 2020 10:52 am

With one product for both adhesion and waterproofing encapsulating, the idea is from the board - ideally penetrating spikes not just 2D - right through to the finish coating

is all one cohesive layer, with the tensile fabric fully embedded inside.

Using multiple discontinuous layers only adhering "2D flat" IMO increases risk of delamination as the composite is stressed and ages over time.
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