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UV in the Shop #2: Finding Flaws in Finish

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Blog entry by Rich posted 02-16-2018 06:32 PM 1788 reads 0 times favorited 3 comments Add to Favorites Watch
« Part 1: Hide Glue Fluorescence Part 2 of UV in the Shop series Part 3: Titebond II Fluorescent to the Rescue »

Another thing that conveniently fluoresces in the shop is finish. Most finishes I’ve tested fluoresce to some extent — some more than others. Shellac, for example, glows a bright orange. Oil based products exhibit different shades of yellow-green, and waterborne products have a slight bluish hue to them. The real significance of the effect is that it varies with the thickness of the finish on the surface. This allows you to quickly locate flaws in the finish very easily. These are often not visible except at certain angles, so finding them generally involves trying to reflect light at just the right angle, and feeling for them with your fingers.

Fortunately a good UV flashlight will make these really stand out. Here is a table leg that was sprayed with Arm-R-Seal. To the naked eye, it looks flawless. The UV light shows that there are in fact a couple of minor flaws that need to be touched up.

Finally, here are examples of some topcoats where I deliberately created thicker areas to show the effect.

Shellac:

General Finishes High Performance Top Coat (waterbased):

Waterlox:

Arm-R-Seal:

I’m sure that if you spend the $10 for a good, powerful UV flashlight, you’ll be surprised at what you’ve been missing.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner



3 comments so far

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TungOil

1006 posts in 615 days


#1 posted 02-16-2018 06:37 PM

This is interesting Rich, I never would have thought to use UV light to check some of these things. Thanks for posting this, you have got me thinking…

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

View Rich's profile

Rich

3548 posts in 710 days


#2 posted 02-16-2018 10:19 PM

It’s a perfect companion to the König finish planer you did the review on, Tung. That planer is a lifesaver. It and their Special Scraper have totally changed how I do topcoats now, and the UV light offers a good way to give it a final look. With the planer and UV light I was able to flatten that sag in the table leg finish in just a couple of minutes and rubbed it out with burlap. It’s pretty much undetectable now.

I already had the Mohawk version of the scraper blade, but the König one is much nicer. I also got several of the Lee Valley ultra-thin scrapers the last time they had free shipping. They come already ground square, but without a burr turned on them, which makes them ideal for scraping finish flat and smooth.

If I stopped and wrote you every time I think about how much I appreciate the review you did, you’d get sick of hearing from me.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View TungOil's profile

TungOil

1006 posts in 615 days


#3 posted 02-17-2018 02:46 AM

Glad you found it useful, I know I find that thing a real life saver too!

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

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