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Question DetailsAsked on 4/29/2016

Should I use a Marine spar varnish to seal outdoor cedar siding?

I have an outdoor porch and shower stall that is finished in cedar. There was originally a sealer applied but after 4 years it is flaking and pealing off. I have used a marine spar varnish on my mahogany front doors and they still look great although they get very minimal direct sunlight. Would marine spar varnish also be good for my cedar? Thanks

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Sorry - that would be one of the highest-maintenance choices - probably requiring stripping and recoating every 2-3 years as it cracks - annually in high sunlight areas. Ask anyone who owns a boat with varnished surfaces - a constant maintenance issue. Because cedar has a lot of natural oils that bleed out, it will also bubble and peel on cedar and redwood and similar woods.

Another important factor is that if you put a basically waterproof (paint, silicone sealer, heavy-body stain) finish over a porous wood like cedar, there will inevitably be water getting into the material along the edges and ends and also through the micro-cracks that inevitably form in any solid-surface coating. Unfortunately, while the water wicks in real well through small cracks and pinholes when it rains - and REALLY gets sucked up along any exposed edges and ends - once under the surface finish it can not effectively evaporate through the same small cracks because it has to escape by vapor diffusion which does not move significant amount of moisture through tiny openings, so the siding ends up staying wet and rots. Granted, slower with cedar or redwood but will still happen, and staying damp promotes cracking and splitting of the wood as well.

Therefore, if looking for good appearance for a long time (so not leaving bare, which will gray out) what you want for a cedar surface is basically the same thing you want on a wood deck - a penetrating finish that soaks into the pores and waterproofs by making the pores waterproof rather than putting on a coating, and it should either be a UV blocking stain (which darkens it substantially) or if a clear finish have UV blocking agents incorporated in it (not as effective). And because of the oils in the cedar, it has to be specially formulated to not bubble in contact with the oil. However, any such finish will require cleaning and recoating every few years regardless. Sikkens Cetol SRD (for vertical surfaces, they also have a Cetol deck finish) and Olympic (several stain and clear products for sidings and for decks) both make good cedar finishes.

Another alternative, which definitely darkens the wood, is a penetrating oil designed for cedar/redwood. In the old days log oil and tung oil were the norm (and of course linseed oil) - today there are other penetrating oil products designed for cedar that do not heavily darken it. Try 2 coats (with proper drying time between coats - commonly a week to a month depending on drying conditions and temperature) first on a scrap or obscure spot to see if you like the amount it darkens the wood. Again, Sikkens, Olympic, Cabot, and BASF all make good ones. My recommendation - stay well away from any linseed or safflower oil based ones because it turns black with mold in short order - mold just LOVES to feed on plant (as opposed to tree based) oils.

Oh - Penofin and Valspar also make popular cedar penetrating finishes, though I would preferentially recommend the ones listed above. BASF and Sikkens would be my preference but can run $70-80/gallon, or about double or even triple Olympic and Valspar and such.

One thing to remember - the cleaning has to be meticulous if clear-coating or staining because any variation in the underlying wood porosity or smoothness will cause significant variations in the wood color - and cross-grain pressure washing is especially noticeable. (Pressure washing is generally a BAD idea on soft siding woods - almost all providers will get at least some cross-grain streaking or cutting of the wood).

Be sure to start in the most obscure spot (back wall or under raised deck) to see what it it going to look like after it cures a few days before doing the entire house, because basically once a finish is on you are stuck with it - you will never get it out of the wood pores if you don't like it. Another reason many people leave cedar and redwood bare.

On the silicone penetrating finishes - in theory you could totally waterproof the wood with them - in practice they do not last at all well in sunlight, so I recommend against them. Especially Thompson's Waterseal - my personal opinion is you might as well just spit on the deck for all the good that does after it has been on a few months. And on cedar it can blister as well.

You don't say what KIND of sealer was put on originally - if silicone, any oil based finish will probably not stick worth beans, so you might be stuck with a silicone sealer reapplied every year or two. If you peel some off, if brittle most likely an oil based stain or sealer. If fairly flexible and sticky or slimy feeling likely silicone. You can also put some water on it - if silicone you should still see at least some balling/beading up of the water after you thoroughly wet it, other type finish will generally sheet free of water. Also - generally (not universally) the chips would not dissolve or soften and become flexible in paint thinner if silicone based.

One thing on the finishing - obviously weather/sun exposed surfaces have to be treated if you don't want them to gray out on you. But hidden or sunlight protected "back" surfaces should not be coated, especially on horizontal deck boards. Leaving the bottom untreated provides a free surfacxe for evaporation to allow water that gets into the wood a surface to wick to and evaporate from.

Answered 4 years ago by LCD

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