According to the labels on the cans of exterior-grade clear finishes in paint stores and home centers, maintaining the appearance of wood appears to be a simple task: Simply brush one of these products over a mahogany doorway, a redwood deck, or cedar siding to keep the wood looking as new as the day it was cut. Will a clear coat finish seal the cracks in wood properly?

The truth is that protecting wood from the weather without resorting to a thick coat of paint is difficult. Some finishes peel like sunburned skin or are little more than solvents that evaporate into thin air when exposed to water, ultraviolet light, and mildew spores while attempting to cope with wood’s inclination to shrink and swell.

Fortunately, developments in coatings technology have resulted in new formulas that, when properly maintained, live up to their promises.

Clear finishes operate in one of two ways: they either build a hard coating on top of the wood or they penetrate it. Film-forming materials, including traditional varnishes and modern urethanes, are unrivaled in their ability to bring out the beauty and depth of a wood surface while also protecting it from wear and strain. However, they can be difficult to apply and are never forgiving of neglect: The film will begin to crack and peel if not carefully sanded and recoated every one to three years, and will eventually need to be removed down to bare wood. Penetrators, on the other hand, protect wood by soaking into its fibers, so they don’t peel or need to be scraped or sanded; instead, the finish simply goes away. They do a better job of allowing damp wood to dry out than hard coatings, and they may be recoated without much surface preparation. However, even the best ones require as much reapplication as film-formers and do little to protect the wood surface from filth and damage.


What Causes Cracking?

What’s the source of the cracks? If the finish isn’t old and brittle, it was probably applied incorrectly. Applying the finish too thickly (common with conversion varnish), employing a hard/brittle finish over a softer/flexible finish, or choosing the inappropriate finish for the job are some of the most typical mistakes that lead to cracking.


More UV Resistance

The most effective approach to prevent UV in the long run is to use pigment. It isn’t noticeable if it closely matches the natural color of the wood and is used in small quantities. If you use too much, the coating may darken the grain and resemble a stain. Nonetheless, a new family of pigments created for automotive paint in the 1970s provides UV protection without sacrificing clarity. Transoxides, or transparent iron oxides, are particles crushed to such fineness that they physically fit between visible light wavelengths. In effect, the visible photons pass almost undetected, while the majority of the shorter UV waves reflect back and disperse before reaching the wood.

It’s all well and good to know how to finish work. The difficulty is that manufacturers closely guard their ingredient lists, so consumers have no way of knowing how much or what kind of pigments, UV absorbers, or preservatives are in each can. To make matters worse, the labeling regulations are ridiculously lenient. The term “preservative” on a label, for example, simply means that the government has authorized the fungicide’s relative safety and accepted evidence that it killed some organisms at whatever concentration was tested. Manufacturers are not required to utilize this concentration in their formulae, nor are they required to fulfill any overall effectiveness standards.

What to Use Outdoors

Nothing surpasses the real thing when it comes to the warm, welcoming appeal of a freshly polished front door. The ideal type of spar varnish for outdoor use is marine spar varnish. Linseed oil (pressed from flax seeds) and alkyd resin are the main constituents (made by reacting linseed oil with alcohol and acid). The film is flexible enough to move with the wood as humidity changes cause it to shrink and swell. Every year, sand lightly and apply a fresh layer to keep the wood looking nice. Otherwise, the varnish would break and become brittle. And you’ll have no choice but to scrape it all off and start again once that happens.

How Can Cracks Be Fixed

Should you attempt to repair it? If the object is a rare antique that is more valuable in its original finish, you should leave it alone and store it in a climate-controlled environment to avoid external stresses. If it isn’t a valuable antique (which most aren’t), you can either leave it alone or fix it. Consider that a cracked finish will allow moisture, humidity, and dirt to seep into the crevices, causing the wood to enlarge and deteriorate, causing the finish to peel. Leave it alone if you like the way it appears and there’s little risk of further damage from water or moisture entering into the cracks. If you want to give it a new look or protect the wood from damage caused by normal use, go ahead and make the necessary repairs. In most cases, you’ll need to scrape off the old finish and apply a new one.

In circumstances when the original finish is lacquer or shellac, re-amalgamating is a repair process that can save the finish. Lacquer and shellac can be re-dissolved and poured out using the right solvents since they produce a film by evaporation of the solvents and no cross-linking occurs.


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