Here are some Q&A's from my Woodworker's Journal Ezine replys.


Saving Spongy Wood?

I am restoring an old oak rocking chair that has been outside in the weather. I have sanded down the chair, removing the old dirty surface. The oak wood seems to be unusually soft on the surface. I plan to use spar varnish on the new surface. Is there anything special that I should do to prepare the chair for finishing? - Bill Barker

Tim Inman: That soft surface is evidence, more than likely, that the wood fibers have been broken down by the sun. Ultraviolet light and weather destroy everything. UV light attacks the lignin that holds those wood fibers together. Then, the actual cellulose of the fibers begins to decay. Water just hurries things along. Sometimes one can sand off the ruined surface and expose a nice new one to finish and enjoy. Other times, the damage is too deep. Varnish alone will not "consolidate" the surface and make it hard enough for use. There are things like epoxies and polyesters which can be used. Caution: Test first to be sure this will be right for you. The damage you're seeing is part of the history of that chair. Maybe you could keep some of the damage, and tolerate the imperfection, without doing further damage to the wood.


--------------------------------------------------------------------------------

Sun-burned Woodworking?

The recent mention of the pyrography book reminded me of something I had read recently – solar wood burning. I can see some benefits from using the sun and a magnifying lens to burn wood, such as low operating cost and the burn point will glide effortlessly over opposing grain and other bumps or uneven surfaces. However, I am wondering if the concentrated spot of light will be damaging to eyes, and what sort of precautions may be reasonable to take before trying this. Your thoughts are appreciated. - Lance Gardner

Tim Inman: As a boy, I remember fondly learning of the energy of concentrated beams of light focused by my grandmother's magnifying glass. What power I had in my hands. The ants, however...

Yes, solar power can be concentrated to the degree needed to burn wood. Laser cutters are, after all, nothing but concentrated and highly specialized forms of light. If I were interested in doing artistic woodburning, and willing to spend the time to execute my designs, I think I would opt for either a laser, or a conventional burning knife setup. Skimping on my tools has never worked to my benefit, in the long run. It might be fun to try the solar option, though!


--------------------------------------------------------------------------------

Right Speed for Bowl Sanding?

I have read Betty Scarpino's article on Lathe Sanding Secrets [on woodworking.com] and would like clarification on one part of it. When hand sanding a bowl, she recommends sanding at a slow speed. Can you be a bit more specific, please? 300 - 500 - 800 ? After turning a project at 1,800 rpm or better, everything to me seems slow. Appreciate your help and keep up the great work! - Gary Kostick

Tim Inman: Well, this might give everybody a headache, but let me just say two things about speed and sanding: One, sanding speed is a very personal preference based upon the operator (you), the wood type, the size and shapes of the object in question, and the results you're after.

Two, rpm gets all the press, but it is actually fpm that is more important. Here's the headache part, because math gets involved. R stands for revolutions. This is easy to measure and understand: how many times per minute does the object spin around? The answer is the same for a thimble as it is for a big punch bowl. Easy to count. But the really important issue is F, which stands for feet per minute. This varies depending upon the diameter of the object being turned. In other words, how many "feet" of wood surface actually show up to be sanded in one minute? If the object is a thimble, with a diameter of .5 inches, then the fpm would be something like 65 fpm at 500 rpm. On the other hand, if you're turning a punch bowl with a diameter of 18 inches, then the fpm would be about 2,355 feet at 500 rpm: over 35 times as much sanding surface in the same time spinning on the lathe. Considered another way, the thimble represents running a piece of sandpaper the length of your shop and back in one minute. The bowl means you need to run that same sandpaper almost half a mile in one minute!

Fpm is important to understand because not only is there more surface to sand on big projects, the faster that surface passes under the sandpaper, the hotter that sandpaper gets - and heat is ultimately what kills sandpaper and spoils good surface prep. Use the slowest speed that does the job the way you like it; that's my rule.