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  #1  
Old 12-19-2019, 04:59 AM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
Location: Melbourne, Australia
Posts: 246
Default Micro Consistency

Hi!

I am almost ready to make the "seam" along the edge of my elevator tip fairings.

Steps so far:
- bonded alu strips inside for the rivets to bear against
- a gross shaping to help make it look right
- the "joggle" edges have been sanded back
- the edge of the elevators have been taped up with packing tape, then a single layer of electrical tape (to help achieve a consistent gap), and finally
- the whole gel coat has been sanded with 80 grit.

I am ready to mix some micro to spread over the whole thing in order to both finish the edge to a consistent gap, and also fill any exposed weave and pinholes that have shown up from sanding.

Q: what sort of consistency am i looking for in the micro to achieve this? I assume it needs to be kind of loose so that it flows into the holes? Or do i make it thicker to help make the edge against the elevator skin, and push it with a card (hotel card etc) into the holes in the fibres?

Do you think i need to use flox or a couple layers of cloth at the front to make the large gap where the counterbalance skin is curved and the tip is not?

I appreciate any advice that is available - this is my absolute first go at fibreglassing! (actually enjoying it so far - it seems mistakes are easy to fix).

Here are some overview photos of the left tip:

https://photos.app.goo.gl/cezWUr3jQQtdrCK89


https://photos.app.goo.gl/tfECVnSU6SLjf8bk9

and here is one showing the sanded back edge ready to take some micro to help make a nice edge:

https://photos.app.goo.gl/H7vVxc1Qo5ty8Yfe8
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May 2020

Last edited by TASEsq : 12-19-2019 at 05:07 AM.
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  #2  
Old 12-19-2019, 06:07 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 9,207
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Trent, I'm not sure sure what you're trying to do here, so this is general in nature...

Microballoons can be combined with mixed epoxy in any proportion. There is no right or wrong, only a consistency which gets the job done.

Generally, we use the maximum possible proportion of microballoons ("dry micro") when correcting surface profile. As a practical matter, it's the stiffest mix which will still spread smoothly. There are two primary reasons to use a stiff, dry mix; the finished product will be lighter, and it will be easier to sand.

I might use a medium mix if I was wiping micro into small holes, divots, and or scratches in a foam form. The key here is that it wipes in easily.

The best known use for wet micro is filling the pores in foam prior to adding an overlying fabric, wet-on-wet. The high proportion of epoxy allows the mix to flow into the voids in the foam.

As the mix proportion tips toward more epoxy and less microballoon, the cured product will be heavier and harder to sand, but somewhat tougher.

Micro is not a pinhole filler. Holes, divots and scratches, yes. Developing a surface profile , yes. But not pinholes. In fact, when sanded, micro is a collection of very tiny pinholes; sanding cuts open all the hollow bubbles on the surface. They are nowhere near as large as the typical pinholes in fabric layup surfaces, but they are there. Spray a coat of primer, and unsealed micro will be immediately obvious.

Many builders use polyester resin based commercial body fillers on emp tips. It's not quite as tough, nor as adhesive, but it works. In addition to, or replacing microballoons, commercial body fillers may contain talc, ground styrene plastic, and a variety of other fillers, each mix tailored to a desired consistency and sanding density. Less hollow micro and more solid powder results in a more closed surface after sanding.
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  #3  
Old 12-19-2019, 06:32 AM
BillL BillL is offline
 
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Location: Central IL
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Good big picture from Dan as usual.

A few specifics for judging the cup mix, when mixing with a popsicle stick, the mix will stick to the stick, as the stick is withdrawn from the mix it will:

1. Stretch a little then bead on the tip and begin to slowly smooth and drip off the stick in 10-15 seconds.
2. Stretch a little more, then slowly bead on the cup side and likely not move on the stick. May smooth out, but won't creep off vertical surfaces.
3. Stretch and leave a relatively sharp point on the cup and stick, there is no movement of either.
4. the stick stays inclined in the cup mix. or can pick up the cup with the stick and it won't drop off.

These are 4 points on an infinite scale. Temperature will affect them vs ratios. I like something with a little smoothing action but not a fluid so it will support shear on the vertical. A tiny bit of cabosil goes a long way to shifting to stiffer near the end of a mix.

Get some dixie water cups and mix 5 grams of resin at a time to experiment a little. Some black pigment can be added (sparingly) to create a gray color that is easy (er) to see when sanding.

As in any body work, be sure to put on enough material so no original surface is left after sanding.
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Last edited by BillL : 12-19-2019 at 06:35 AM.
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  #4  
Old 12-19-2019, 07:31 AM
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Bob Martin Bob Martin is offline
 
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I know this sounds confusing but.

I was taught to apply raw epoxy to the fiberglass part before adding/applying micro. The "dry" micro isn't wet enough to soak into the glass structure by itself. Wet micro will be wet enough. The key here is you want bonding to occur.
I will sometimes drill small holes like 1/16" to help with bonding as it is getting covered/filled anyway.
Once you get close, I switch to fine/finishing automotive bondo, because it is easier to sand in very thin layers and feathers out smoother than micro.
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  #5  
Old 12-19-2019, 08:23 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
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Location: Clinton, Indiana
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Default +1 for Bob's Epoxy priming comment

Regardless of the coarseness of the surface (raw glass or not) I prime with epoxy then stir micro in the balance of the resin. This is especially relevant to "dry micro" application when there is not a surplus of mobile resin in the mix.
PS, I also find dry mico does not spread well ( re Dan's comments), so I lay a piece of poly film over the rough deposit and shape it nicely with my hand, credit card, roller or what evever works. Leave it til cured and it looks pretty good for minimal sanding.
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Last edited by Larry DeCamp : 12-19-2019 at 08:36 AM.
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  #6  
Old 12-19-2019, 08:47 AM
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DanH DanH is offline
 
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IF priming the sanded surface with neat mixed epoxy, use a small piece of cotton rag and rub it on to the surface, so there is no wet liquid, just damp. Wet epoxy guarantees dry micro won't spread well, and worse, when sanded after cure, the neat epoxy will form a hard ring around the micro due to being harder to cut.
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  #7  
Old 12-19-2019, 08:48 AM
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MarkW MarkW is offline
 
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Trent,
If I may give you some insight from my poor choice of micro balloon use.
I used fiberglass tape and then some mb for this same area to fill that same gap between the fiberglass and elevator. Since I have been flying for six years now I have paint chips in that area. When I fly through heavy rain I get new chips there. My though is that the mb are too soft. This is not a great area for MB. Yes they make sanding much much easier but you need more strength there.
I will be using chopped fiberglass and some Cabosil in that spot.
Filling pinholes is a different matter.
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