PMF Questions....

Anything to do with mechanical, construction etc

Re: PMF Questions....

Postby lacofdfireman » Sun Mar 31, 2019 8:59 pm

John61CT wrote:The more experience the PMF expert has around here, the more they emphasize each project is its own experiment.


I’m learning this go round for sure. But the process was pretty simple. I am however going to have to do a trim board on the bottom of mine since I wasn’t pleased how the bottom edge turned out on a few areas of mine. Other than that feel really good with how it went.


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Re: PMF Questions....

Postby Nodrog » Mon Apr 01, 2019 12:46 am

Hey all- I will chime in here, having had a little experience with a lot of things....If you think of the essential nature of plywood, the veneers are cut by rolling the log and slicing a thin layer off....this exposes a lot of borders or junctions between the winter and summer grains...it is at these junctions that changes in humidity, and the effect of intense sun exposure for example followed by at times, intense moisture or at least humidity due to rain or fog, raise the edges of these "grains".
I believe the cloth in the PMF serves to apply pressure, like a continuous clamping action, to minimize movement of the "grain" and stop checking in the finish. An example- I had a harbor freight trailer that I used to haul work supplies. 4x8x 3 high...made a lid that was hinged in front, etc. so a lot like a teardrop. Material- 3/8 sheathing (fir) plywood. I just painted the sucker. And let it sit out always....let me tell you, it looked pretty baaad after a few years, especially the top! Checked, oh boy! about 1/2 of the paint left after @ 10 years when I retired it and myself.
But.. PMF has been used on boat decks and tops for about 100 years, don't forget canoes, so...I think there is a reason for the fabric!

Did I make any sense? Later, kiddos.....Nodrog
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Re: PMF Questions....

Postby John61CT » Mon Apr 01, 2019 7:57 am

Makes sense.

Another aspect is, the total thickness of sealant* + sealant-embedded cloth + more sealant layers on top

will be **much** greater than just painting the wood.

*sealant meaning generically, TB2 and/or Gripper plus outer finish paint

So likely useful as a "thickener / filler" even if the tensile strengthening property is not needed as it is with Foamie builds.
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Re: PMF Questions....

Postby gmoulder » Wed Apr 03, 2019 3:01 pm

Hello all,
Am proceeding with my 6' x 10' x 5'-6" trailer. Walls are 1/4" ply inside with composite of 1" x 3" pine and 1" XPS. Walls on cabin are perhaps 60% wood in area and 40% exposed XPS. The roof will be tilted from the rear [ala TAXA Cricket] and be constructed of 2" XPS with 3/4" x 2" poplar stringers. The roof area will likely be about 90% XPS. Work is proceeding rapidly to a point where I must apply an outside covering. Conducted tear-off and beam stiffness tests recently with 2" XPS x 3' with TB3 and canvas on one plank [both sides] and Foam Fusion and canvas on the other [also both sides]. Suspended both planks between sawhorses 3' apart and piled on sandbags. Neither plank failed, although the FF plank had much more deflection. FF did, however, beat TB3 in the tearoff test, actually removed some of the foam during test [I perforate with the wall paper tool]. The TB3 panel was much harder to the touch. Did cheat a little with the TB plank in that I also brushed a 50 / 50 mix of TB and water onto the canvas after the initial placing of the canvas on the foam. Have discarded the idea of using bug screen as the fabric, do not believe the FRP bug screen will integrate into the adhesive like canvas fibers. NOW: have read and re-read all the PMF posts [I think}, and do not see any concensus as to what to put on foam to make PMF. I see a paint being used, a wood glue, no mention of the only adhesive specifcally made for foam to foam and foam to porus material. Some posts use TB on the foam followed by a diluted mixture of TB. Some use TB on the foam with a paint follow-up. BTW, just spoke with the Foam Fusion factory, and they say FF is 100% water proof. Apparently several of the posted systems work, but which works best over time? Would truly love to get feedback from actual users as on the several systems which seem to be popular. Otherwise, I believe I am headed towards full strength TB3 and canvas on the cabin exterior, followed by a diluted TB mix worked into the outside of the canvas with a fiberglass roller, followed by Gripper and then finally an exterior paint. This system because of the large amount of exposed wood on the cabin. The roof [top and bottom] would be FF to attach the canvas to the foam, followed by Gripper to saturate the fibers, followed by exterior paint.
HELP. Please weigh in on these potential applications. Thanks, Glen
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