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Any tips on glassing a model?


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  • 3 months later...
On 03/04/2022 at 16:43, Allan Bennett said:

Thanks for all the info.  I've decided to modify my past technique by sealing the balsa first with sanding sealer, and then perhaps adding a second coat of resin after the first glass and resin has hardened and been sanded.

 

Turns out I didn't need a second coat of resin but, having now crashed the model that I treated in this way, I think I won't be doing it again.  The model is being repaired at the moment, but in every impacted area the glass/resin has lifted off the sealed balsa.  Repaired areas are being glassed without pre-sealing the new balsa, so I'll probably use some "one-coat" emulsion paint that I have to fill in the weave before final sanding and painting.

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1 hour ago, Allan Bennett said:

 

Turns out I didn't need a second coat of resin but, having now crashed the model that I treated in this way, I think I won't be doing it again.  The model is being repaired at the moment, but in every impacted area the glass/resin has lifted off the sealed balsa.  Repaired areas are being glassed without pre-sealing the new balsa, so I'll probably use some "one-coat" emulsion paint that I have to fill in the weave before final sanding and painting.


Ok, but it didn’t delaminate under flight loads, so does that really matter? If it does that in a crash I’m not sure that really matters; after all you are still going to be cutting out any damaged areas and re-glassing either way. Certainly back when all the best DLGs were foam bagged in carbon/Kevlar/glass layups, they always usudto seal the foam before bagging, though exactly how it was done did vary by builder.

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I too used sanding sealer on the balsa before glassing, mine de-laminated following a crash.  After speaking to the dealer I found that it was the sanding sealer that didn’t allow a good base.  He recommended sealing with thinned resin or not at all.  I use a foam roller, the resin is really smooth after that.  A quick sand and a second coat followed by another quick sanding leaves a fantastically smooth, fuel proof skin.  

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Yes, I suppose the fact it didn't delaminate in flight is a good point.  But I would feel happier if it didn't delaminate so easily -- at some points in the repair I'm having to resist peeling back too much before sanding down an edge to bond to the repair glass.

 

My sanding sealer is water-based (because my wife doesn't like the smell of the stuff I used to use).  The repair is going on without pre-sealing, and I'll be applying a second coat if needed.

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1 hour ago, Bucksboy said:

I too used sanding sealer on the balsa before glassing, mine de-laminated following a crash.  After speaking to the dealer I found that it was the sanding sealer that didn’t allow a good base.  He recommended sealing with thinned resin or not at all.  I use a foam roller, the resin is really smooth after that.  A quick sand and a second coat followed by another quick sanding leaves a fantastically smooth, fuel proof skin.  


Yep, thinned resin is probably the best way to go, applied with a roller so only the bare minimum is applied. I am pretty sure that was the most prevalent method used on bagged DLG wings back in the day.

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My two peneth from glassing. Glass shouldn't lift from balsa even in a crash if properly applied without taking a lot of the balsa with it.  Priming the balsa with various primers or sanding sealer before appling glass will weaken the bond as the fillers in the primer are weak. In sanding sealer its just talcum powder used to fill the grain of the wood that really weakens the bond. To lay the grain of the balsa apply a thinned coat of dope 60/40 works OK.  Don't flood the balsa but apply just enough to wet the surface and raise the grain . Once dry sand with 240 or similar grit paper. The thinned dope will still allow the epoxy resin to soak into the balsa a little and form a proper bond when the glass is applied.

A tip from a friend long ago was to lay the glass matt out flat and give it a light coat of plain hair laquer and and allow to dry, this will stop the weave from moving when stippling or rolling. The  laquer quickly softens when resin is applied but holds the weave long enough while the glass is applied. 

Don't forget that the glass supplies most of the strenght to the finished item and the resin supplies most of the the weight. A heavier grade of glass is often better than extra resin .

On all the models I've glased I used 25gsm glass for most of the frame and some times heavier grade in high stressed areas, a cheap disposable roller and a old brush for application . Beware of using too cheap a brush or you will spend most of your time removing bristles from the resin. Any wet areas (puddles)had toilet tissue or kitchen roll laid on to soak up the excess but using a roller helps prevent the puddles forming in the first place.

 I've tried many methods suggested by friends and on forums etc but what works for you and gets a hood finish is always the best 😉.

Edited by Engine Doctor
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11 hours ago, Engine Doctor said:

My two peneth from glassing. Glass shouldn't lift from balsa even in a crash if properly applied without taking a lot of the balsa with it.  Priming the balsa with various primers or sanding sealer before appling glass will weaken the bond as the fillers in the primer are weak. In sanding sealer its just talcum powder used to fill the grain of the wood that really weakens the bond. To lay the grain of the balsa apply a thinned coat of dope 60/40 works OK.  Don't flood the balsa but apply just enough to wet the surface and raise the grain . Once dry sand with 240 or similar grit paper. The thinned dope will still allow the epoxy resin to soak into the balsa a little and form a proper bond when the glass is applied. . . . 

Yes, my observation is that it seems the sanding sealer has weakened the bond.  Priming with dope doesn't appeal to me though, because of the smell.  So maybe a light coat of thinned epoxy is the answer.  So, what does one use as a thinner; pure acetone?

 

. . . A tip from a friend long ago was to lay the glass matt out flat and give it a light coat of plain hair laquer and and allow to dry, this will stop the weave from moving when stippling or rolling. The  laquer quickly softens when resin is applied but holds the weave long enough while the glass is applied. 

Don't forget that the glass supplies most of the strenght to the finished item and the resin supplies most of the the weight. A heavier grade of glass is often better than extra resin . . . 

The glass moving as I apply it is not a problem I've encountered.  On curved surfaces especially it's beneficial/essential for it to move to get around the corner tidily.  I generally use 25g cloth, with 50g where I feel it's beneficial.  But doesn't heavier cloth require extra resin as well because of its extra thickness?

 

. . . Beware of using too cheap a brush or you will spend most of your time removing bristles from the resin. . . . 

I've always used cheap disposable brushes.  To help stop the bristles falling out I crimp the metal tube that holds them with a pair of wire cutters before using them.

 

. . .  I've tried many methods suggested by friends and on forums etc but what works for you and gets a hood finish is always the best 😉.

Amen to that!

 

12 hours ago, Don Fry said:

Did it delaminate, or pull the top layer of balsa off? I would expect, if you look at the underside of the glass it’s balsa coloured. 

 

Close inspection reveals a few specks of balsa on the underside, but it's mainly epoxy-coloured, sort of brownish, so basically very poor adhesion.

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Engine Doctor, buy some iso propyl alcohol (propan2ol). If you look on the data sheets for epoxy resins, the cured product has better final properties with this thinner than propanol, or especially the shorter carbon thinners, ie ethanol and methanol. Not the end of the world, but better to optimize. Our stuff can challenge  structural limits. 

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As is real acetone it's all rubbish today, however when I normally use epoxy resin its at least 22°c so it is fluid enough to properly penetrate balsa or even samba skinning, then under a  sheet of bubble pack draped over it as a 25°c + oven ( in the sun outside more than 30°c) it does end up thin enough to properly penetrate the wood surface.

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I tried the MRM finishing epoxy because I wished to spray my Stampe cowl with it. Supplied in tiny bottles only it would have been ridiculously expensive to do a whole model. To my surprise they recommended 50/50 cellulose thinners for spraying; this works with skinning resin which is basically the same thing. Once tried epoxy thinners but it was very slow to evaporate off and more than 10% inhibited setting altogether, same went for meths. Acetone is great for cleaning pots and brushes but I would not try it as a thinner. IPA is usually sold by the gallon as a nail varnish remover but this probably has a water content of around 10% so not pure enough.

You certainly would not wish to thin down skinning resin anyway otherwise the cloth would not adhere. West System resin is by far the best bet.

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  • 2 weeks later...
On 20/09/2022 at 10:15, Martin McIntosh said:

I tried the MRM finishing epoxy because I wished to spray my Stampe cowl with it. Supplied in tiny bottles only it would have been ridiculously expensive to do a whole model. To my surprise they recommended 50/50 cellulose thinners for spraying; this works with skinning resin which is basically the same thing. Once tried epoxy thinners but it was very slow to evaporate off and more than 10% inhibited setting altogether, same went for meths. Acetone is great for cleaning pots and brushes but I would not try it as a thinner. IPA is usually sold by the gallon as a nail varnish remover but this probably has a water content of around 10% so not pure enough.

You certainly would not wish to thin down skinning resin anyway otherwise the cloth would not adhere. West System resin is by far the best bet.

IPA will not touch nail varnishes, ?Acetone will.

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The following is from Fibraglat.com, courtesy of one of our members who has used this method.

Material:

24g min of fibreglass cloth, non starch type.

Two part finishing resin/hardener. 

Small set of scales

Mixing cups and sticks

Brushes, small and medium

Sharp knife

Rubber gloves

220-280 grit wet and dry

400 finishing grit

800-1200 polishing

Methylated Spirit

1st Coat impregnate wood and stick cloth to item.

2nd coat fills weave on fibreglass cloth

3rd coat gives a smooth finish ready for primimng

4th coat if needed, look at weave to see if it is filled.

 

Method.

1 Cut cloth with 1-2" overhang of object to be covered

2 Wet and dry item. Do this using dry 200 grit  remove dust

3 Mix two part resin 50-50% 200g R 200g H

4 Add same amount of Meths so 1:1 ratio, mix until liquid is not cloudy

5 Apply resin mix on pre positioned glassfibre. Put a good quantity in the middle and work outwards from this. Ensure you go around the edges. Check for bubbles.

6 Leave to dry for a few hours or overnight.

7 Remove overhang with sharp knif  and wet and dry the edges. Remove dust.

8 Repeat stages 1-7 on other side of item.

9 Mix resin 50 - 50 only using 50% of meths apply to both sides. Leave to dry overnight then use 200 and 400 grit for finishing of use this wet until surface is ready for priming/painting.

 

I have seen the results of the above and all are excellent 

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Thanks for that method Zflyer.  Although you've seen it work in practice, some of it seems a bit unnecessary to me:  My resin (correct mix for my brand is 5:1 with hardener, not 1:1) is thin enough to impregnate the wood, stick the 25g cloth to it, and almost fill the weave.  So I don't understand why one would want to dilute it with meths, which others have suggested is not the optimum thinner for epoxy resin anyway.

 

I've just had to carry out a repair the other week and simply applied more of my regular resin as the first coat than I normally would, and it sanded down nicely when set.

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I guess different folks employ the same method with differing recipes. I do note, but failed to put it in the above instruction, that Kingfisher Aviation were supplier of the epoxy being used. Like to think that epoxy resin is all the same but we do know different.

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There is a wealth of information regarding epoxy (and other resins and materials) on the R&G website.

I have had a lot of success moulding and laminating with their resin L with various hardeners over the years, and stocked up before European purchase became a bit more difficult.

Even if you don't want to try buying from Germany, their website is worth a bit of time looking through just for the free (and in English) information :-

https://www.r-g.de/en/list/Resins/Epoxy-resin

 

 

Dick

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