Making a flexible sanding spline... need some input

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cblink.007

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Hey all, I am skinning the 25% scale proof of concept prototype of a design of mine, and at a point where I need to be thinking about finishing!

This model, as a simultaneous dress rehearsal / laboratory for moldless fabrication, is made with known foam/glass/epoxy matrix. It is unorthodox by "Giant Scale" construction standards, but it has been a great learning experience so far, and alot of fun! Ultimately, this will fly as a dynamically scaled model to verify the results of our simulations, giving us the final vote of confidence to complete the full scale (2 seat) design and get to work building it!

Anyways, I can't seem to find a flexible sanding block anywhere around my area, and given the compound curving that ultimately defines the unique shape of this beast, I need one.

Any ideas out there? I found a stiff 12" sponge mop head for starters...

I am open to ideas!

In the meanwhile, here are some pics from the project to date:
 

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Hot Wings

Grumpy Cynic
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I use something like this:
Sand.jpg
This isn't exactly like the one I use but is the same idea. It is my 'go too' sanding block of VW bugs. You can both curve and twist at the same time. You can build something similar with a piece of thick plexiglass and some wood handles bolted on. Use spray adhesive to attach the sand paper if you don't already have adhesive sandpaper strips.

For final hand sanding a piece of neoprene wet suit (or similar) material works well as a backer.
 

cblink.007

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Thank you! I have a solid sanding block for 2D/flat surfaces... great for fairing leading & trailing edges, but not for any compound curves!

Hopefully the local Lowes isn't closed. Go figure, you have some downtime to get things done and all the stores are locked up!
 

Victor Bravo

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Piece of .025 or .032 aluminum with sandpaper glued on it makes a flexible spline for single plane curves.

Speaking as an ancient, crusty, curmudgeonly old model airplane builder who has done this a thousand times, 3D curves are often best done with a piece of sandpaper in the palm of your hand. You can make little masking tape loops to put a finger through, to hold the sandpaper from shifting.
 

wsimpso1

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Sanding sticks will vary with surfaces, and for those big compound surfaces, my guess is you shall end up with a bunch.

Around my shop I have everything from T sections made of Melanamine coated shelf stock to foam sticks with various concave and convex curves of several thicknesses to give different compliance and shape. You can even make a T section of foam cut to 1/2" thickness to be stiff the long way and soft torsional. My wings and tail surfaces are straight tapers, so straight sticks dominate. If your wings have compound curves, you will be well served with sticks will curves on them in both the long axis as well as the short axis. Make to suit. Some folks even make sanding sticks from pieces of plexiglas.

Billski
 

dog

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there are foamsanding blocks, some with the grit
all the way through and others with it only on the surface, paint/drywall section.
then your local auto supply will have all sorts of stuff for the curves on cars, boats etc.
talking to a boy racer could be profitable
 

n45bm

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I never thought of the foam pipes...thank you! What will be sanded is the typical micro-epoxy paste mixture...ultimately finiahed with the "epoxy wipe" method. I hav had great results on test pieces so far!!
Pool noodles make more sense to me, and you can get them in different diameters.
 

Marc W

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I used to do fiberglass tooling for a living. My job was mostly building plugs. I have very little use for a flexible sanding block. If you have a high spot to knock down the flexible block will knock down the high spot and also the low areas around it. Then you have to fill the low areas and sand again. Better to use a hard block and apply it where you need it.

As a professional, I didn't have time to fill and sand and fill and sand ad nauseum. My favorite tool for shaping was a 7" Sioux body grinder with a 36 grit disc. I can shape to within a few thousandths with a body grinder. Next tool to remove ripples from the grinder was either a small rubber hand block for smaller radii or a long board hand sanding block. A long board air sander is handy for leveling primer but isn't much use for shaping. For inside radii a piece of bar stock or pipe works well.

One mistake that I see amateurs often make is to use to fine a paper. The idea is to cut the high spots down - not polish them with fine grit paper. I usually use 40 grit for rough shaping and then go finer to get the final finish. It does depend on what you are sanding. For sanding your foam parts 80 grit is probably aggressive enough. You do develop a feel for it.
 

Hot Wings

Grumpy Cynic
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You do develop a feel for it.
THAT is the essence of it all.
Some people just never seem to be able to develop that feel. Others, like us, can scarf splice a sheet of ply (any thckness) by hand with our favorite tool.
And thus the great debate about moldless composite construction.........

I never really liked the rotory grinders. For compound curves straight line air file (36gr) followed by the flexable sanding block (80gr) and I'm ready for finish prep.
 

User27

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+1 for a hard block - to change the shape you must use a hard block and coarse paper, I usually use 60 grit and change it often. For compound curves use a smaller block and go diagonally across the curve. Pieces of wood work well and are easy to profile where necessary.
 

dog

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if you need bigger sheets of coarse or very coarse
sand paper, the sheets that go in flat plate flooring
sanders might be of interest
I glue them to a flat board to use in metal work or anything else I want to flatten by hand
 

Marc W

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The Sioux body grinder wouldn't be much use for shaping foam. For one thing it takes some muscle to handle it and a lot of practice to learn to control it. It really shines for shaping fiberglass or when you are shaping a part with different hardness materials in it. I never used used foam back in the day. I used wood, fiberglass and bondo. Properly handled, it will cut through all three materials evenly. Probably overkill for foam. I was a husky lad when I was building plugs. I am to old and weak to use the 7" grinder anymore. I use a little Makita 4" grinder now. I still have the touch though!

If you do want to try a grinder, this is the way to do it. You use a sweeping motion - like waving your hand. Start by sweeping the grinder back and forth above the part and gently lower it until it just touches. Keep moving it - don't stop! You want to use the outer part of the disc but not the very edge. The disc will flex and grab when it hits and you have to be sensitive to that. You adjust the pressure you are putting on the tool according to the hardness of the material you are grinding and how much you need to remove. You will have to experiment a bit to get good results, but you can do precise work with it after some practice.
 
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