minwax polycrylic on top of minwax oil based stain OUTDOORS


I thought I would share this. 5 years ago I finished a mahogany transom with minwax ipswitch pine and then topcoated with polycyclic. For some reason it has held up well. Even though it is sold as an interior product. Wood that got the polycrylic without the stain shed the polycrylic in a couple of years and the wood turned grey. Could the answer be the stain sealing the wood which allowed the polycrylic to stay on the wood?
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I came across this posting and he appears to have some similar experience.
http://groups.google.com/group/rec.woodworking/browse_frm/thread/f9bc391afa2b325c/d2cce9aecda368a1?lnk=st&q=polycrylic+minwax+exterior&rnum=3&hl=en#d2cce9aecda368a1
'The plywood dart-board cabinet puzzles me. I painted the back with exterior latex. Then, I applied a few coats of clear Minwax Polycrylic (water-based *interior* finish). The Minwax label said not to do this. Don't ask me why I did it -- just a crazy afternoon playing with the chemistry set, I reckon. Anyway, after six years of brutal Florida sun, rain, storms, hurricanes, humidity, etc., etc. -- that Polycrylic film looks brand new! I have no idea why; I had no right to expect that kind of performance. But there it is. Go figger. '
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sdowney1 wrote:

I don't know if this is relevant in this particular situation or not. But when we stain a door, we stain it as opaque as possible to keep the UV off the wood. ALL clear exterior finishes are problematic when used alone, and much research by (not sure it was them) the Thermatru corporation as well as others has proven that there is much less damage to the wood when UV is blocked, not just mitigated. Uv damaged wood dries out faster, shrinks, and moves which destroys the top coat. As the top coat gets older and more brittle, it is less elastic and starts to break down with movement.
Bearing this out, doors we have stained an opaque color and finished last much better than the ones we just put a toner in the finish and apply.
Robert
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sdowney1 wrote:

At a guess, your 'ipswitch pine' was a penetrating oil stain, and even mahogany can weather a bit better when oiled. Minor porosity in the varnish would allow some moisture in, and the oiled wood sheds that small amount of water while un-oiled wood might soak some up. The wet/dry/wet/freeze cycling can open up major cracks in the wood, and cause it to shed a finish.
Spar varnish (real spar varnish, intended for real spars back before polymer science...) was extra oily and gummy (and never really dried out). Folk who tried it on indoor furniture found it stuck to clothing... you may have replicated its virtues without the drawbacks, by doing those two layers of finish.
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