X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [69.171.52.140] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0.4) with HTTP id 887189 for lml@lancaironline.net; Sun, 18 Dec 2005 12:31:14 -0500 From: "Marvin Kaye" Subject: Re: [LML] Re: Body Work To: lml X-Mailer: CommuniGate Pro WebUser v5.0.4 Date: Sun, 18 Dec 2005 12:31:14 -0500 Message-ID: In-Reply-To: <20051218053648.62398.qmail@web81212.mail.mud.yahoo.com> References: <20051218053648.62398.qmail@web81212.mail.mud.yahoo.com> MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for daniel newland : Hi James The correct length and stiffness varies based on the surface to be faired. Like for a wing like a Lancair (tapered trapezoidal shape with all chord lines parallel so that say 30% from the root to the 30% at the tip or 60% at the root to 60% at the tip has straight lines betwen them) sanding parallel to the chord length, a 6' long rigid board would be good. But an eliptical wing like a Spitfire would require a 6' board with some flex, like maybe 1/2" thick aircraft grade Fin birch on a 6' length. ANY variation in contact with these boards that will assume a fair shape is either a high or low. One trick I use to achieve a perfectly fair surface is to mold release wax up a batten like an aluminum bar, aluminum channel or whatever is stiff enough and bends in a fair shape, attach it next to the surface like with clamping or taping, then pack epoxy putty under it. Really pack it in. Once hard, you remove the batten and what is left is a perfectly fair shape that requires almost no sanding. This saves TREMENDOUS amounts of time and sanding. If you do enough of these datum lines, then fill in between with putty, you can't help but have a perfect surface. So just like a sanding board, these battens have to be the right length and stiffness for the job. But I digress (often). I don't like sanding in only one direction and in one plane. You also need some sanding parallel to the sections or frames (butt lines). To do the wing here, a 30" - 36" batten that is flexible is needed. Like maybe 1/4" plexiglass would be good. You shouldn't have to do a lot of sanding this way but there may be some variations in the sectional fairness that need addressing. Then for a constant diameter fillet for example, I would tend to make a sanding block that was solid wood, like maybe pine and radius it to match the fillet. The length only needs to be one sheet of sandpaper long. For changing radii, you might try a little hard block sanding with the custom pine blocks and some neoprene or neoprene/rubber composite for blending. Tools like orbital sanders can be a great help but they do NOT fair a surface, they smooth it for finishing. A long board can make a surface fair but it might look like Hell when finished because it was not smooth. Conversely, a surface sanded smooth with an orbital sander can lead to a gorgeous gloss paint job that shows all of the waviness in the unfair surface. You have to delineate between fairing and smoothing. So the larger the surface, the larger the board you should use. Always use the longest board and the coarsest grit you can for fairing. Also, change paper very often! The abrasive should be SHARP, not just rough. A good abrasive will cut the surface almost like a plane or Surform. It should never just rub the surface off. The absolute best advice I can give is to use sharp (fresh) sandpaper when fairing. It does a much better job spanning the lows and taking off the highs. The finer the grit and the more dull the abrasive, the more work you have to do and the worse the results. I want to make a point here related to what I said earlier about the difference between smoothing (getting ready to paint) and fairing (overall contours). Fairing is taking care of variations in shape over large areas and as such, should only be done with coarse grits intended to remove a lot of material. Once you are getting ready to paint, you will ABSOLUTELY need to apply another coat of primer and then hit it with the orbital and fine grit (like 180 or 220 grit). The temptation is to think that because the surface looks even, all the scratches are removed but this is virtually never the case and you only find out about the scratches after you apply the gloss. So don't even try, just apply another layer of primer (I like to add about 1/2% red, blue or green pigment to contrast the under layer so I know when I'm through), and then smooth it with 180 on an orbital. Anything finer than what the paint will level and fill in is a waste of your time and paint. Most linear polyurethanes easily hide 180 and 220 grit scratches so normally 180 is the finest grit I ever use before painting. Laquer tends to need finer grits but I doubt anyone uses that on their planes. Two tools other than the orbital sander I want to mention. Normally, the Black and Decker stuff is kind of so so but I do really like their "Mouse" sander (about $40 as I remember). It is great! I also have a Dremel contour sander. I have made 4-5 special sanding shoes and stick self adhesive backed Velcro on to these custom shoes. You can get into corners and around special shapes like you never thought possible. So I vote those two tool as extremely handy. As for orbitals, I have used very many but I have never used any sander better than Porter Cable. If that name is on the front, it is a GREAT sanding tool. They just have sanders down to perfection. I love my Milwaulkee tools and I decided to try a Milwaulkee orbital. It was a very good tool but just not as good as the PC. Last, 3M makes some really good velcro backed long board paper. It is 33" x 4" I think but you can always link a couple for big sanding boards or cut one off. I buy this stuff by the sleeve. It's expensive but saves a lot of time and for velcro backed pads, I cut them to fit the sanding shoe and save a BUNDLE. So you can but precut sandpaper for the Black and Decker mouse sander, or cut your cost by 90% and cut your own from the 3M velcro backed paper. Now you know everything I know about sanders, fairing and smoothing. Dan Newland