Making a flexible sanding spline... need some input

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wsimpso1

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Here is how I am doing it. Billski's Fiberglass Bird starting at post 25. Really dry micro, 36 grit paper on big hard boards. In those posts, I cite the best references I have found on this.

I know that the toothed spatula method is recommended by (among others) my friends at Gougeon Brothers, I go for the driest micro I can put on and go thicker than you need it. I tried the toothed spatula on some of my plug work. It was tough to profile. Where you do not need to take off much, it sands easily, where you are taking off a lot, it gets hard to sand. This is because the resin tends to sink toward the substrate - where you are sanding down closer to the substrate, the sanding gets tough, while the stuff you are not trying to sand so much of is still sanding more easily. It still does this with dry micro, but if it is really dry, it does less of this. Heat gun passes on the back of the drywall knife helps the dry stuff go on smoother. Put a good coat of dark epoxy primer between the part and the micro, and you can see when the micro is getting thin - time to stop sanding there.
 

wsimpso1

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Does anyone use the notched trowel method? I notice a lot of micro balloons are used, does anyone use West Z410? It's my filler of choice, very light and easy to sand.
Scaring me. West 410 filler is phenolic microballoons, and has a reputation around airplanes for two undesirable traits:

First is that with some epoxies, it will preferentially absorb hardener and inhibit curing. There is a rule that came from RAF way back: Mix resin thoroughly before adding microballoons - do not add unmixed resin to micro.

Second is it has a significantly larger thermal expansion coefficient than quartz or glass micro - I have a couple pieces on the pickup truck where i used this stuff as a general filler. The scratches that I tried to fill barely show, but where the flaw was deeper and the fill thicker, it bulges up in hot weather and shows craters in the cold. Ugh. I have it as final filler per Wayne Hicks on my flaps and ailerons - I still have to place the one with the most filler outside in this heat to see how bad it is.

Quartz/Glass microballoons do not tend to have either of these characteristics. The glass does not absorb resin, and its expansion coefficient appears to be very close to that of the glass-epoxy composites underneath it.

Billski
 

Royal

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Jun 8, 2020
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I have done all the body work on all my restoration with Paint Stir Sticks, Yard Sticks, 3x1" premium pine boards, and aluminum extrusions.
I try very hard to not ever use foam or anything that's soft. Using a foam block allows it to ride down into the low spots and high spots. With the paint sticks it will keep the shape of the stick and only sand the high spots down to the low spots. You can get a box of paint sticks really cheap. Some are stiffer than others. Use the stiffer ones on a slightly curved surface. The very flexible ones are for those really curved surface.. There are a lot of other techniques I use while using these tools. I focus on the flat spots or slightly curved surfaces first. Places like the intersection of the fender flares to the fender and body like on the Bronco I DO NOT build the fillet till the fender is straight and the flare is straight. Here is a video I made on doing inside curves with sockets.
I have not sanded the peanut butter stuff you guys use. I know the filler i use has silica balloons in it as well but maybe not as many.

I should make a new video on how I do all of my body work.
 

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wizzardworks

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Well I made planer curved solid wood sanding blocks. Sanded at 45 degree angles with 50 grit. By turning the sanding blocks they matched
all the surfaces. My fuselage looks like a large BD5 over 17 feet long. There is a thread detailing the construction with plenty of pictures showing the fareness. The thread is "How I am building a fuselage mould" This mould is also a female compound surface mould set up with a matched fit top and bottom so most of the conctruction is in the open bottom tub. The main wing is one piece mounted to a truss inside the cabin yhat alsop mounts the engine, landing gear, and fuell cells for gas. The engine is a 5 rotor Mazda with my own endplates for parallel cooling out of MFS aluminum with silicon carbide nodules for wear resistance like a lawn mower engine. Porting was also flow modified. The wings are all corrugated spar and one piece. The main wing has strakes, taper, and winglets and a combination of 4 airfoils NLF (1) 215 and NFL (1) 416
and Eppler 266. ANYWAY I VOTE FOR RIGID CONTOURED SANDING BLOCKS WITH 50 GRIT and finish polishing with an orbital 5" Ryobi sander to clean up sanding scratches wiped with automotive glazing compound.
 

User27

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England
This is good for gentle curves, velcro bottom to take paper. But the velcro does introduce a small amount of compliance which makes it half way toward a soft block.
https://www.mirka.com/uk/File-Board-70x400mm-Grip-53H-Flexible-Yellow-8391150111/

Microballoons - we use red balloons 90% of the time at work. Most definitely mix resin & hardener first, then add whatever filler you choose. We use L285 + H285/6/7 90% of the time, and use H286 in about 80% of those mixes. No problems with soaking up hardener. We test blob every mix to ensure proper curing.
 

Geraldc

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Nov 12, 2011
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nz
I have a flexible block like this one.
Supplied by Eastwoods and others.
If you want to make one it is a thin metal bonded to a foam block.
The one shown has 3 rods inserted lengthways to stiffen it. Remove rods to make it more flexible.
1614811239686.png
Different ones I have made are rubber pads that I glue velcro onto that you can buy in rolls.
The best paper I have been using is 3m cubitron.
 
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