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How much Micro is too much Micro?

drone_pilot

Well Known Member
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In order to get vents level and a consistent gap on nose ring, I had to move the bottom cowl about 1/8" forward. This created an "underbite" on the outboard cowl inlets at the horizontal seam. I'm totally new to fiberglass but I have been reading a lot in these forums. Is it acceptable to build up the top cowl inlet with micro and sand to match the bottom cowl or would I need to use something more structural here? Also, how do I need to prepare the surface, etc?
 
Yes, that is exactly what you would do. If you have a rather long transition area try laying one layer of glass over the top of your build-out. Do not forget to gap the cowling to the fuselage when the micro is fresh. I use the narrow width of a tongue depressor for this.
 
I would, at the least, sand a good bevel on the part and use flox instead of micro balloons. It will provide more strength. Then a layer of cloth behind the extension would be good insurance.
 
An 1/8" of micro will be fine for this simple fill application. Perfectly routine. The key to long term happiness is ensuring a good bond with the pink glass, i.e. surface preparation, and that just means sanding the pink glass with 80 grit until it is entirely dull.
 
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Also

As Dan said this will be fine. I always lay a layer of light glass cloth over anything more than 1/16" thickness. I keep some 2 ounce cloth around for that. Use ply peel over that and the transition is easy to sand to perfection.
 
It is hard to tell from your photo but I also added a lip to the bottom half that blocks the air from going through the split. It also provides a nice transition as it will get painted and help hide the split line a little bit.
 
An 1/8" of micro will be fine for this simple fill application. Perfectly routine. The key to long term happiness is ensuring a good bond with the pink glass, i.e. surface preparation, and that just means sanding the pink glass with 80 grit until it is entirely dull.

What he said.

Also, Darwin's suggestion about a light weight cloth overlay is good.

Additionally, if you plan to install a fancy pin cover like this one from Aerosport http://aerosportproducts.com/index.htm you might consider waiting a bit and doing all your glass work at once.

cowlcover2.jpg
 
Composite materials

If your getting some thickness to the micro add in 10-20% of flox, that will add the tensile strength missing from straight micro yet still sand nicely.
Tim Andres
 
Security??

......Why?

There are several reasons. The overlay of cloth will reach beyond the actual seem or edge of fill material. Less chance of any chipping of the fill.

In areas where there is a seam between dissimilar materials such as the glass pieces on the elevators/stabilizer I also bond with a layer of cloth. My plane had over 800 hours when sold and no sign of problems. I have seen planes that used only fill and there was cracking at the seams.

Maybe it is not necessary but it has worked for me in not only composite model applications but on the "ride in" stuff too for decades.
 
Why

In my opinion micro is to soft when your flying in rain. Past experience has shown "pitting and chipping" of the paint where micro was in the ram airstream. Add some flox or put some glass cloth over it and that will stop the pitting. Micro is good on a surface that is not in the direct line of attack from water. Just my observations!
 
I would use straight micro. You are able to sand micro, but flox is very difficult to sand. If you use fiberglass, you are still gonna need micro to make it look good after cure.

The entire outside surface of a fiberglass airplane is contoured using micro, but the leading edges of EZ's don't pit or chip in the rain. Your paint will chip off long before the micro.
 
Maybe it is not necessary but it has worked for me in not only composite model applications but on the "ride in" stuff too for decades.

I hear ya brother. Nothin' wrong with liking a belt AND suspenders ;)

In my opinion micro is to soft when your flying in rain. Past experience has shown "pitting and chipping" of the paint where micro was in the ram airstream.

Either you had a paint problem, or I'm in big trouble. My cowl and intake scoop has been fending off bugs and rain about 450 hours.
 
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