# Exterior vs. interior finish



## crquack (Oct 10, 2008)

Can wipe-on poly be used as an exterior finish? If not, why not? I was told today that it would fail "due to the UV light". What is the mode of failure? What is the time scale?

My immediate thought was that a hard polyurethane coat is more likely to crack with wide temperature and moisture variations.

If you wanted a finish that would work inside *and* outside what would you choose?

BTW another thing I was told today was that an exterior finish used inside is more likely to turn amber.


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## Dejure (Jul 27, 2009)

You want what is called a long oil finish. This means more oil was used in the formulation, which softens the final finish, leaving it more flexible and tolerant of shifts due to temperature and moisture content changes. However, more oil in the formula produces a softer finish, thus one less durable under heavy handling or foot traffic.

As noted, exterior formulations have UV shields to impede damage from sunlight. They may also have a mildicide to reduce problems with moss and mold.

While fading from UV is undesirable in most instances, I am less worried about it than cracking and loss of adhesion caused by the different rates of contraction and expansion of the finish and protected surface.

No law says you can't experiment. For example, you can saturate the surface with a thinned hardening oil (pure or polymerized tung oil does not provide a food source for minute critters) before applying a long oil finish. The oil will never crack like a varnish or other hard surface coat. 

You can even add a small percentage of oil a finish to become familiar with the effects of different levels of oil in a finish on wood. For example, you could add twenty-five or thirty percent poly to tung oil or linseed oil, or the same percentage of hardening oil to a poly and see how they did on some test wood under adverse conditions (you'll still have to wipe off excess after it's allowed to set for a few minutes). It wont dry overnight, but the results might surprise you too. Especially if you are prone to sticker shock when shopping for good quality exterior finishes, which normally start at fifty to seventy bucks.


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## jlord (Nov 16, 2009)

You could use a spar urethane such as Helmsman Spar Urethane. It is made for exterior conditions & is a modified alkyd resin. A spar urethane will give general protection, as well as flexibility, & UV protection. For a higher quality product try Epifanes. It contains phenolic modified alkyd resins & a tung oil base..


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## crquack (Oct 10, 2008)

Helmsman Spar Urethane is what is commonly available locally and it is advertised as interior/exterior finish. The logical choice would be to get a pot and be done

However...

Brushes of any sort are my enemies. I like the wipe-on method, so I thought I would ask.


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## Capt Splinter (Dec 30, 2008)

I bought a little wooden sign at a country fair for my wife. It was painted with no other protective finish. In my un-trained mind, I thought, why not put a coat of poly over it to "protect" it. Well, not knowing any better, I coated the whole thing with interior wipe on poly. Within two weeks, the sun started to turn it yellow, and after one month, it was completely ruined. I am now experimenting with the exterior spar poly myself. I am also wanting to learn more about shellac for interior projects. Good luck finding what works. I think this forum has great people with great ideas.


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## crquack (Oct 10, 2008)

Thank you. Yours is the sort of experience I was wondering about.

Before I use a finish on metal I experiment. I have a way of speeding up corrosion so I can condense days into months (within reason). I am not sure I could replicate your experiment with wood as easily.

Oh well, I better get used to brushes...


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