I built a park bench out of white oak. I wanted to prevent the oak from changing color due to weathering and finished it with a water based marine grade UV stabalized varnish. The bench has been exposed to direct sunlight and rain for about 4 months and it is apparent that the varnish is not preventing moisture from getting under the finish. The oak lumber in a lot of places is begining to change colors. The color change is greatest in the part of the white oak lumber that has small pores exposed to the surface. The varnish remains attached to the entire surface and shows no signs of pealing. What can I do to this piece to preserve its original nonweathered oak color. Is there a marine spar varnish that will protect the oak and prevent a change in its color? Can this bench be recoated with a spar varnish that will stop any future color changes to the wood?
I googled on spar varnish and did not find much help with this problem. Any help you can give me would be appreciated.
Just out of curiosity, I wonder if a water-based polyurethane would hold up? I know the Spar varnishes have a long chain which enables them to stretch more and be more resistant to UV. Since boat builders us Spar, I guess that may be way to go. Still, I wonder??? Hydrocote, Resistane
There was a recent article by Michael Flexner (sp) in one of the mags that answered this question. Simply the answer is ...no. The UV will continue to cause the wood to discolor. You can stain (oil based - penetrates better), or paint but any clear topcoat is going to allow the UV through.
Joe, I read the other answers {4 at this time}. I have a feeling that the other responders may have skipped over what maybe THE REALLY important phrase, " . . .The color change is greatest in the part of the white oak lumber that has small pores exposed to the surface.".
First, is the wood *REALLY* WHITE Oak? I don't mean any disrespect, a lot of RED Oak is sold AS 'White Oak', and he two look very similar. {A simple test, if you have any scraps left. Take a small piece with the grain running lengthways, put one end in some water and blow through the other end. RED OAK is so 'open tubed' you will get bubbles in the water; like using a straw.
Both 'Oaks' are notorious for being a problem to finish, especially with an epoxy. A trick is to wipe them down with Acetone . . . IMMEDIATELY BEFORE laying down the first coat of finish. {Varnish - typically thinned as much as 50%; but 20 to 30% should do very well}. I would NOT use a SPAR varnish on anything that will be sat upon. It never *really cures*; it's not supposed to. It will always remain 'soft'. There are a number of 'Marine grade' short-oil varnishes that have a UV additive . . . I don't think ANY {for EXTERIOR use} are 'water clear' but have the traditional slight 'Amber' tone. {Pettit, Interlux, West Marine {house brand}, ZAR, etc}
How long a Varnish finish lasts when exposed to UV depends DIRECTLY on the NUMBER of coats{about 6 being a typical 'norm'}, and the integrity of the finish {an old trick is to keep a nail polish bottle, filled with varnish, handy. As nicks are noticed, they are 'touched up'}Normally, the finish is 'refreshed' on an annual basis {actually depending on the UV exposure and 'handling'}. This usually involves a gentle scuff sanding and a fresh {sometimes thinned 10%} coat flowed on.
For YOUR piece, {IF you want a varnish finish} I would recommend the following:
1} Thoroughly sand of the old varnish to get a smooth surface; think of the coats you have as 'fillers'.
2} Get some good UV additive varnish . . . NOT SPAR.
3} Get some containers so you can thin quantities of varnish.
4} Thin a coat by about 50% and flow on a thin coat . . . you are trying to fill the pores of the Oak.
5} Wait 24 hours then sand with 220 grit and wipe with thinner dampened lint free cloth
6} Thin a coat about 30% - flow on - 24 hours - sand with 320 grit, wipe
7} Thin a coat about 10% - etc, etc. - sand with 400 grit, wipe
8} [OPTIONAL - another 10 % coat - depending on pores - sand with 600 grit, wipe]
9} Flow on UNTHINNED coat
If you have been patient, it should look like glass. If not, apply a couple of coats of past wax with a very fine 3-M pad and buff well.
For the *ULTIMATE* finish . . . when starting from 'new' . . . seal the pores & surface with EPOXY before applying the UV Varnish. Only about 3 to 4 coats are needed and the 'base' is fairly well impervious to everything.
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