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Posted for daniel newland <compositeguy@sbcglobal.net>:
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
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