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Does un-varnished plywood weaken with long storage ?

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keith103

Well-Known Member
Joined
Nov 12, 2013
Messages
134
Location
Aurora, Denver,CO
Last year, I purchased a Minimax Pt 103 project, built from factory supplied kit. The project was half done, with fuselage and tail unit completed. Work stopped on the project 20 years ago after which the kit and half finished airplane were mostly sitting in the basement in FL / AL.

I am presently building the wings ( about to start assembling the spar).

The whole project ( finished items as well as unfinished raw wood stock ) have not been varnished ever. Since they have been stored indoors, there is no water damage. The spruce pieces ( spar caps etc ) look good and I am using them as is.

The ply wood sheets, most of which are cut to size, are visually not damaged, and there is no de-lamination anywhere, but since they have been sitting without a protective coating for 22 years, I am wondering if they may have lost some strength. Had they been varnished and then stored, I would have felt more re-assured.

Also I have reason to believe the second owner , who never worked on the project, has taken reasonable care of the project while in storage in Alabama. Everything was kept organized and the fuselage was itself suspended from the ceiling in his huge basement. Just by looking at the way things were kept organized when I took delivery, I felt confident that project was well looked after. However, the region where it was built / stored are high humidity locations ( Florida and Alabama ). Had it been Colorado or AZ, I really would have had no reason for concern.

The first thing I did after bringing home the project to Colorado, was to varnish the 1/8 plywood walls and flooring enveloping the cockpit to prevent further weakening of the cockpit structure.

Presently my dilemma is whether I should use new plywood for the wings ( spar web, leading edge skin etc ).

Just by holding and feeling and bending slightly with my hands, I felt the new plywood I purchased recently from Aircraft Spruce is slightly more rigid / firm than the old plywood in the project kit, of similar thickness and grade. I also tried poking a sharp knife into the new and old plywood with comparable pressure. The older ply had a slightly wider incision than the newer ply. Just a bit more, not a whole lot more. I don't know if that means anything at all, because it is such a crude way of testing.


Is there a way to test the strength of old plywood ? Also, does plywood weaken with long storage ?


Thanks for any suggestions / tips.
 
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