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205 vs 207


davidpm

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I have some mahogany trim on a Spindrift I would like to finish varnish.

Most folks recommend coating with one coat of epoxy before varnishing.

I usually use west 205 hardener for gluing applications.

I notice that they sell the 207 hardener which is specifically for clear applications.

Do you think it is worth the trouble and expense to get some 207?

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If you were using a really light colored wood like holly, and wanted it stay as white as possible, then yes.  With mahogany, I wouldn't bother.

Most folks recommend coating with one coat of epoxy before varnishing.

Hmm, I guess I'm not part of "most folks"  When you are trying to build up a resilient waterproof coating for a hull it is surely a good idea.  But I don't think it does a heck of a lot for trim.  It does go on a lot thicker, so it will fill grain faster.  It has no UV protection, and will only last as well as your varnish work protects it. 

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One or two coats of epoxy will not have enough color in them to affect the looks of your wood. Go with the "normal" epoxy/hardener mix. Most varnish has more color in it than epoxy. The epoxy is better protection for the wood and the varnish, in turn, protects the epoxy.

All of the boats I have built have epoxy under the varnish on the bright work. They look great after quite a bit of use and exposure to the sun.

Steve

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207 is visibly more clear then 205, 206 and 209, but more importantly it doesn't darken as quickly with age as most other resins do. On naturally finished mahogany (no stain) there will be a noticeable difference between the two in lighter colored areas, if side by side for comparison, but other wise most wouldn't be able to tell the difference. As Dave pointed out on light colored woods this is much more noticeable, possibly objectionable, but dark woods, not so much.  It's the aging effects from UV that is the biggest reason to use these especially clear resins.

Even the best UV inhibiting clear finish (varnish, etc.) will eventually let enough UV down to the epoxy and the wood itself, causing both to darken or change color. With enough UV exposure, epoxy proceeds to get a chalkiness to it (which can be polished out, if there is enough film thickness), then browning progressively until turning a slate black color. 207 is less prone to "depolymerization", which is the physical break down of the intertwined and linked epoxy molecules (actually one big giant molecule when fully cured). All plastics do this to some degree with UV exposure.

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