Paul Jacobson
11/28/2023 1:35 PM
The best exterior varnishes will be found at boating supply firms.
They are not cheap, and may be only slightly better than what can be found in the Varathane or Minwax spar varnish lines, seen at Menards, HD, and Lowes.
The big killers of clear finishes are sunlight and moisture.
With paint there is an opaque, light-blocking, pigment mixed in which prevents UltraViolet light from affecting more than the surface of the finish. Damage happens to the exposed surface, and slowly progresses through the paint layer. This might take 10 to 30 years.
With a totally clear finish ( like interior water-base polyurethane) the UV light passes completely through and destroys the bond in about a year of direct sunlight.
The ‘Spar’ or exterior varnishes contain a UV blocking filter, which has an amber or yellow tint. This breaks down over time, so it needs to be reapplied. Typical life is two years. Some of the better marine products will last three or four.
On boats, which are rarely docked in the shade, reapplying varnish to exposed woodwork is considered to be an annual task. If the base is still good, a light sanding to scratch the surface ( and offer some ‘tooth’ for the bond) is simple and fast and a single coat gives another year of protection.
If you wait three years though, usually you’ll have enough damage that you’ll need to strip off the old layers, get down to the wood, and start again. The sanding and stripping is a major task, and a reason why old boats, which have not been maintained for years, are sometimes given away.
Some people are lucky and get two years between recoats.
Heat and moisture cause the varnish film to expand and contract. The spar varnishes are slightly softer and more flexible than interior varnishes. They stretch a little bit and tolerate the expansion and contraction.
If the wood is sealed with epoxy it will have fewer issues with moisture. If it is made of red oak it will have more moisture related issues over time. Particularly if the endgrain is not sealed.
Rather than a varnish, an oil-based stain is popular on cedar and redwood. Those which are ‘solid’ or ‘semi solid’ contain finely ground opaque pigments which soak into the wood pores and fibers. A very popular brown pigment is iron oxide, or common rust. The pigment offers a slight blocking of UV light, extending the life of the coating, and unlike the amber tint in spar varnish, rust never breaks down, no matter how strong the UV. Stains may last 5 to even 10 years before needing to be recoated. Many do not need a varnish over them. If the door is sheltered from rain (behind a storm door) the finish may last longer.
You can find boat finishes by Cabot, Pettit, Total Boat and other brands online. In the Indianapolis area there is a West Marine on the NE side, on 82nd street ( just a few miles from Rockler and Woodcraft. They have very good quality boat finishes in stock.
Hope this helps.
Paul Jacobson
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