PMF Experiment

Finishes, paints and coatings

PMF Experiment

Postby Sandro » Sat Oct 12, 2024 8:59 pm

I am in the process of gathering materials and components to build my first teardrop.

After doing a comparison of skinning options, I was leaning towards PMF for the following reasons:

1.- Ease of application compared to fiberglass.
And
2.- cost.

I am not afraid of fiberglass, but living in the Pacific Northwest and building a teardrop in a garage could prove problematic. Temperature fluctuations, humidity and dew points are things to watch out for when working with fiberglass/epoxy.

This teardrop will be parked outside with a tarp cover. It rains almost all the time here. Cars are always wet from rain or dew. Moisture is inevitable.

I figured I would do an experiment with PMF to find out how well it would keep moisture out. And to prove it to myself that it would be an adequate skinning option for my climate.

So, I prepared a block of OSB. If moisture gets in it will show up in this material. I followed the PMF instructions: applied Titebond II to the OSB surface, then applied the canvas. Waited for it to dry and applied diluted primer followed by full strength primer. Then applied 3 coast of exterior latex house paint to saturate the weave. For each coat of primer or paint I waited at least 12 hrs for it to dry and recoat.

After applying the last coat I waited 48 hrs for the paint to cure and drilled a hole. I placed a bolt through the wood and added weather stripping caulking. Then waited 72 hrs for the caulking and paint to dry.

Next, I placed my block into a bucket and placed a weight to hold it down under water. I added some red food coloring to see if it would migrate from the exterior into the OSB substrate.

Waited 8 days and rinsed off the block and dried it with a towel.

Initial inspection showed no signs of dye intrusion through the PMF membrane. So far so good.

I removed the bolt and no water/red dye had made it past the caulking. That was good.

Then I cut the block using my jig saw. The wood appeared moist but not wet. I did not see any dye where I made the cut.

Then proceeded to check the PMF adhesion to the OSB. It peeled easily with just little force. Moisture had migrated into the membrane and compromised the adhesion. There was a noticeable amount of red dye that had made it past the membrane into the glue/primer.

In conclusion at least for my teardrop, I will be using fiberglass. Maybe in dryer climates PMF will be perfectly adequate. But for the Pacific Northwest mushroom patch where I live it does not seem like a good choice. The moisture intrusion would just cause the cotton fibers to mold and rot. Same for the wood. Perhaps I used the wrong glue or the wrong paint. I know 8 days under water is a long time and it may not be a fair test for PMF. However, It is wet for weeks at a time where I live.

Comments welcome.

Wood block preparation:
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Inspection after 8 days in water:
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Last edited by Sandro on Mon Oct 14, 2024 6:37 pm, edited 1 time in total.
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Re: Preview: PMF Experiment

Postby tony.latham » Sat Oct 12, 2024 10:29 pm

I will be using fiberglass.


I assume with epoxy resin and not polyester?

Image

Image

By the way, I find fiberglass/epoxy almost as easy as varnishing.

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Re: Preview: PMF Experiment

Postby Tom&Shelly » Sun Oct 13, 2024 6:07 am

Sandro wrote:
Comments welcome.



Excellent experiment! :thumbsup:

Very well conceived. Thank you for describing it and letting us know the results.

Shelly and I only started using expoxy/fiber glass when we built our teardrop six years ago, or so, and, like Tony, we found it reasonably easy to get good results. Our teardrop has spent three summers in the northeast and has not had problems with moisture.

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Re: Preview: PMF Experiment

Postby Pmullen503 » Sun Oct 13, 2024 6:45 am

PMF can also be done with oil based paint over wood. That's how it's been used forever on boats.

Latex paint was designed to be permeable to allow moisture in a wooden building to pass through the paint and not cause it to fail.
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Re: PMF Experiment

Postby Sandro » Mon Oct 14, 2024 6:56 pm

Thanks for your comments.

Interesting point about the permeability of latex paint.

I think I will conduct a second experiment using Oil Based paint just to satisfy my curiosity regarding PMF. I will report my finding here in a couple of weeks.

Regardless of the outcome, I would use epoxy and fiber glass since it is a solid solution for waterproofing and strength.
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