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The paneling darkens from exposure to UV from sunlight. This is irreversable. What you want to do is return the paneling to its former color, or match the light spot to the darkened paneling. If the paneling is smooth you can sand off the UV damaged surface. If it has details/grooves this is not an option. You can experiment with stains. Thin the stains with paint thinner so only a little goes on at a time. Use very little thinned mix on a paper towel and spread quickly. Whatever color the UV damage is- grey/brown- grey/honey- dark honey- etc.- select stains in that color family and mix. Use an eye dropper and count the drops of each color. Adjust the number of drops until you have a stain that is close to the UV damage. Test the mix on scrap lumber that is close to the light spot in color.
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General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next