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Old Mar 23, 2014, 03:28 PM
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dk31's Avatar
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PU vernice + Baby powder as primer

I don't know if this is the right place to ask but anyway.
I ve been experimenting covering foam with light Fiberglass cloth (I tried both epoxy resin and PU vernice).
Right now, I am trying to get rid of the "cloth look" and achive a nice glossy wing surface (before applying paint). I tried the "PU vernice + Baby powder" method and I only get acceptable results when:
1. Mix 1 part vernice and no less than 2 parts powder
2. When squeezing off the excess, I dont push the squeege (spatula) against the wing - just hold the squeege very lightly

Is this the way it works? I 've only tried 2 layers of primer so far and before the above two actions, it looked like 5 layers wouldn't be enough
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Old Mar 23, 2014, 05:29 PM
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I think you might do better by posting in the Painting and Finishing forums.
However, I'll answer the best I can for the time being.
I've Googled "PU Vernice", but cannot find any description. What is it please?
The closest definition I could find was a reference to traditional Violin finishing using a mixture of egg white and Gum Arabic!

Talc is ground Magnesium Silicate rock and as such, is heavy. It is ok for non-flying components such as plugs for mould construction and the like. However, I consider it too heavy for flying parts. For the job you are describing, I have used primer and ""Q" cells mixture with some success. Wet sand in between applications.
To minimise the voids in the first place, a sheet of mylar or another plastic (waxed first) applied on top with some pressure will work. Vac bagging is the most effective way to do this, but you may not have the equipment to do this. There are many vac bagging threads to be found here.

Also, you don't mention what kind of foam you are using?
All relevant details are always helpful for the reader when asking questions on these forums.
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Old Mar 23, 2014, 05:55 PM
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Thanks for your time..it looks like I have messed up definitions
PU vernice --> Polyurethane lacquer

primer and ""Q" cells mixture --> what primer and what's "Q-cells"?

The foam I 'm working with is EPO sanded to a smooth finish. There are no voids - the only thing I want to smooth out is the pattern/weave of the FG cloth (3/4oz cloth).

Here is another thread describing this technique (post #9 mostly) - he just uses polycrylic instead of PU lacquer
http://www.rcgroups.com/forums/showthread.php?t=161218

Last, vacuum bagging is out of the question for the moment...
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Old Mar 23, 2014, 06:07 PM
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Q cells are micro spheres used widely in the composite industry. Go to your local fibreglass shop and they might sell you a small quantity. Otherwise, buy online I suggest.

"the only thing I want to smooth out is the pattern/weave of the FG cloth (3/4oz cloth). "

I would use several application of spray putty with wet sanding in between.
If you don't have a spray gun, thinned nitro-cellulose "stop putty" applications using a scrap of plastic sheet as a screed would also work. It will involve more sanding, that is all.
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Old Mar 24, 2014, 03:23 AM
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Thank you Jim.
I guess the Q-cells/micro spheres are the same material as micro-balloons (AKA glass bubbles). There's a local shop that sells them but I have never tried them so far. I ll check the weight increase with the talc and try the micro-balloons to compare
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Old Mar 24, 2014, 04:28 AM
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Quote:
Originally Posted by dk31 View Post
Thank you Jim.
I guess the Q-cells/micro spheres are the same material as micro-balloons (AKA glass bubbles). ......................e
In practical terms, yes. Just slightly different manufacture and price.
Generally called microspheres......
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Old Mar 24, 2014, 06:22 AM
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You should check in the Scratchbuilt EDFs forum, lots of guys there use WBPU==water based Polyurethane ..

Just do a search for wbpu.


Search this thread too, John is an excellent builder

http://www.rcgroups.com/forums/showthread.php?t=2033785
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Old Mar 24, 2014, 09:18 AM
The Junk Man
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I think the word you are looking for is "varnish", polyurethane varnish.

There are a LOT of combinations of materials used for filler. Keep in mind that micro balloons/micro spheres/any micro bubbles does not usually make a good, ready to finish, final surface. I prefer talc personally. Everyone has their favorite materials and talc is what has worked for me since the days of nitrate dope. Talc works with dope, polyurethane, and epoxy... basically anything. I just like it.

Tom
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Old Mar 24, 2014, 05:08 PM
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There you go DK, Tom likes the talc and also I think polyurethane varnish is what you are looking for.
If I am correct, Tom builds discuss launch gliders (is that right Tom? - maybe I have the wrong Tom?).
The DLG building fraternity have much to teach us about composite construction.
At five years into the hobby, I consider myself about a 4th year apprenctice!

By the way, industrial talc from composite suppliers is cheap and perfume free.
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Old Mar 25, 2014, 12:52 AM
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I think Mylar as Jim suggested may help reduce the look of fabric, just remember to keep it on until the part has cured.
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Old Mar 25, 2014, 02:15 AM
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I think mylar and other films is not a good base for paint. Right now, I am experimenting with the FG cloth/primer/paint combination to see how I can improve EPO in terms of stiffness, hangar abuse resistance and nice finish colored surfaces. I have seen state of art constructions with films and even packing tape, but that's something to try another time

So, my first results show an increase in weight around 5% for every additional layer of WBPU+Talc. Tom, do you have any good recipe for the mixture? (like the 1 part WBPU and 2 parts talc I am talking about in the first post)?
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Old Mar 25, 2014, 03:13 AM
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[QUOTE=dk31;27887552]I think mylar and other films is not a good base for paint. ...................../QUOTE]

The mylar sheet is not left on the part! It is removed after the epoxy cures.
I suggest a read of one or two of the vac bagging threads might clarify this.
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Old Mar 25, 2014, 01:44 PM
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Quote:
Originally Posted by Jim.Thompson View Post
The mylar sheet is not left on the part! It is removed after the epoxy cures.
I suggest a read of one or two of the vac bagging threads might clarify this.
It's clear to me now It's the first time I hear about using a film + pressure on epoxy without involving vac bagging

Thanks again all and everyone for your replies
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