Covering models with GRP.

[Michael Barclay]

Hi everyone. Let me start by saying that I am a newbie at model flying, but as such have been reading and viewing everything I can on the hobby and I keep coming across the same mistake being made when covering with GRP. (It was repeated again in the June issue of RCME).Namely that the advice given is always to lay the glass cloth on the model and stipple or roller the resin through the cloth. Having spent a lifetime in the boat building industry let me say that no one doing high quality work would use this technique when laying laminate on a hull

The correct technique is to lay the cloth over the part to be covered (let us say a wing) then fold the cloth back for half it's length before wetting out the now exposed part of the wing. Now lay the cloth back over that half of the wing and stipple the cloth with a brush. This brings the resin through the cloth from the underneath ensuring that the cloth is fully saturated and bonded to the substrata. Now fold back the second half of the wing and repeat the process on that half.

Finish the job by sponging out all excess resin with kitchen towels.

Remember that first class GRP work requires that the resin has full contact with the substrata (ie no air bubbles trapped in the laminate) and that the cloth is fully saturated but not over wet as happens with a resin rich (weak) layup.

I can assure those who might like to try the correct method that they will not go back to trying to stipple the resin through the cloth from above.

I generally use a one and a half inch brush for the job. Rollers are not necessary on model work.

[Don Fry]

Carry on .

[McG 6969]

Hi Michael,

Having spent a good part of my previous life in the 'full size' composite world myself, I can only second your statement above.

Of course with the introduction of refrigerated pre-preg stuff, vacuum moulding and oven curing, the process has been in constant evolution.

But, with the 'artisan' approach of model covering, the first thing to take care of is the liaison with the underlying 'substrate'. Being it bare wood (sealer required) or even with a second composite layer applied (kept tacky or sanding required if cured).

To be honest, I can't even imagine why some tutorials are showing off a different approach.

Keep doing it your way and be happy with the results...

Cheers

Chris

[Roo Hawkins]

hi Michael you are dead right . I myself used to build carbon sailboards in the 1980/1990s and now build glass and carbon rc scale sailplanes in 1/4 and 1/3 scale and I would never roller epoxy on top of glass cloth with out first adding resin to the wing or fuz first . I all so never use rolls but brushes and a surf board squeege from sea base uk ROO​

[Martin McIntosh]

Just use peel ply on models. Makes life much easier.

[Don Fry]

Thank you for my continued education. Why did I never think that a inch and a half brush is a fine match for 20 gsm cloth. No question of raising the strands?

[Former Member]

[This posting has been removed]

[McG 6969]

I don't really agree with you, Martin.

'Peel ply' is of course a nice way to remove the excess of resin applied to the tissue.

But the whole point is to have NO excess at all which you don't have to 'remove' afterwards. This is just 'spoiled' resin or hard varnish in the case of acrylic stuff being used. Excess resin + peel ply is just an unnecessary waste of money in this case...

Hence the 'substrate first' method explained by Michael. If you still have an excess when handling it that way, it just means you have been loading an excess of resin at first.

Cheers

Chris

[Bob Cotsford]

I suspect that the idea originated back in the dawn of time in an attempt to reduce the quantity of resin needed to fill the weave. The glass wasn't intended as structural element, just as a way to acheive a smooth finish that was tougher than doped nylon or tissue.

[Allan Bennett]

I've done quite a bit of model fibreglassing and am not sure that full-size techniques are necessarily relevant to our models. For one thing, I can't imagine using a 1.5" brush; mine is about 3/8", though I do agree about not using a roller to simply roll resin onto the cloth.

I simply lay the 25g cloth onto the model surface and smooth it down, then stipple the resin onto it repeatedly, thus ensuring that it has penetrated through and into the cloth. Repeated stippling is often required when going round compound curves, and to get the cloth into corners. I don't have any problem with bubbles in the finished article, nor do I have any excess resin needing to be scraped off with a credit card, as is often mentioned, or sanded off afterwards.

I use the same technique when using heavier glass or carbon cloth (up to 150g) to laminate cowls and the like and find that by taking time I can ensure that the resin has fully penetrated, and soaked into, each layer without any excess.

[Martin Harris - Moderator]

The way I was taught was to dribble a tiny amount of resin onto the centre of the cloth and work outwards with a credit card - it's amazing how far it goes. No brushing or rolling involved and the resin is fully worked through the cloth onto the pre-sealed surface with no excess under the cloth which might be a possibility when trying to draw a quantity of resin from the substrate. Any tiny excess is removed with kitchen towel. No doubt when used in other applications where weight isn't so critical and areas are much larger, the OP's method would guarantee full adhesion without the minute attention to detail that the credit card method gives.

A tip I was given was not to use the lightest grade of cloth - it simply uses less threads per inch, leaving more resin between them and actually works out heavier...I can't recall the weights involved but something to be aware of when choosing your cloth.

Edited By Martin Harris on 09/05/2018 22:27:09

[Mike T]

I freely confess to not being an 'expert' here, but I can't agree with the assertion that the glass isn't intended as a structural element in Glass-Reinforced Plastic!

All the various online/magazine guides I've read refer to the need to keep the resin deposited down to the minimum required to wet-out the glass. Most guides refer to dry application of the glass (or using a tack spray of some sort - which I personally have problems with*) and putting a sparing amount of resin on top, worked in by rollers and/or 'squeegee-ing'.

However, in my limited experience I've had the best results brushing a coat of resin on to the surface, then using a brush, stippling and teasing out as necessary to draw the resin through and mopping off with paper towel as required. If you get it right, the towel mopping should be minimal.

* I have a DLG with glassed balsa tail surfaces. The finish has wrinkled in the sun, due, I suspect to an excess of 'tackifier' being used at the factory preventing the resin from bonding the glass to the wood.

[Tim Flyer]

On my current build of my LA7 I brushed on the resin and used an old credit card to remove excess. The credit card worked well . Glassing is usually simple job in models as they are small and I have used both “resin on top of cloth “ and “resin on surface “ methods . It’s a good idea to glass the tail plane before fitting too! As long as there are no bubbles and the cloth is properly wetted and excess resin removed there should not be any difference. I must say that the credit card finishing works well and squeezes out excess resin well. There are many ways to do it and still achieve good results.

[Cuban8]

Sit six aeromodellers around a table and get them to discuss glassing and finishing techniques...........you'll probably get six different opinions, all of which will be correct.

[Stuart Z]

Hi

No expert at all but use the glass cloth and spread with a “credit card” and get good results. Well, good enough for me and have had positive comment. Will be trying the foam roller on my next build. Just to compare.

BUT, we all have preferred ways of working and we should keep note of other methods as different applications may require different approaches.

S

[Bob Cotsford]

Posted by Mike T on 10/05/2018 00:15:08:

I freely confess to not being an 'expert' here, but I can't agree with the assertion that the glass isn't intended as a structural element in Glass-Reinforced Plastic!

All the various online/magazine guides I've read refer to the need to keep the resin deposited down to the minimum required to wet-out the glass. Most guides refer to dry application of the glass (or using a tack spray of some sort - which I personally have problems with*) and putting a sparing amount of resin on top, worked in by rollers and/or 'squeegee-ing'.

The resin may create a glass reinforced 'plastic' when applied to glass cloth but it was never intended to make an airframe stronger when it first started being used as a base for a paint finish as I first heard it. It was used as a fairly quick, chip resistant and easily flatted basis to slap some colour onto, hence the ideal being 15-30gm cloth back then. This was in comparison to using nylon or tissue attached over sheet using dope. I'm not talking moulded wings or 100gm wing joining tapes, just simple painted glass finishes.

What the process has evolved into is another matter and if it's used to skin a foam core then yes, it's structural and needs to be bound to the core tightly. However as the discussion was why did the article suggest adding resin to cloth and not vice verce I thought I'd point out the reasoning that I first heard 30 or 40 years ago.

[Mike T]

Confused, Bob. Going back to your original post, did you mean that the resin was not intended to be structural and just a substitute for a dope or lacquer finish? If so, I'd agree, and that the glasscloth in that case just serves to reduce the amount of resin required. I've certainly never found it as ding-resistant as some people claim!

[Bob Cotsford]

Mike, I meant the glass/resin was not meant to be a structural element of the model, just an overlay forming a basis for a painted finish, as you say a substitute for a dope finish. I guess the epoxy (or polyester back in the day) resin was just being used as a filler in the glass cloth. As for ding resistance, I haven't tried epoxy resin/glass cloth on a large area but I've used Poly-C/glass cloth a few times and would call it more chip resistant than ding resistant. Still better than tissue and cellulose though.

[Denis Watkins]

Lads, in structural engineering parlance, page 1 suggests that adding a soft material, to a brittle material results in a increase in strength and toughness, not seen in either of the additives.

Soft glass fibres, and resin became fibre glass, and became cars, caravans and radomes on planes.

Paxolin, is impregnated cloth, and carbon fibre we know, for helicopter rotor blades

Where Carbon is one of the softest materials known to man, the manufacture produces the lightest strongest material.

In Michaels introduction, he mentions the most important part of the manufacture using resin, is "Wetting"

An engineering term for surface adhesion

The lower surface, coated, and additional fibres coated, ensures a tough new surface during the bond

It can be structural, as in aeroplane rudder surfaces, boats, bridges, and fishing rods, the list goes on

[Geoff S]

I always use my wife's credit card to spread resin It makes for a very inexpensive finish.

Geoff

[McG 6969]

Thanks for your post, Denis.

An example of clarity for all. The secret of 'strenghtening' lies in the adhesion of the different parts together.

Just like the different kind of 'wooden ply' you are all using without questioning about 'why'.

I've been laminating very soft 3mm Depron between two layers of 0,8mm birch ply. Have a test and you'll be totally amazed with the obtained strength.

So, if your fiber adhesion is too 'dry' it's less effective, if it's too 'wet' you loose on weight, of course, but the excess of resin makes it also too 'brittle'. And that's a shame...

Apologies to Mike T, but I can't understand people declaring that a proper glass covering isn't even more 'ding' resistant. Sorry...

Cheers

Chris

[Don Fry]

Yes Denis, but glass, has no strength in tension, if you have broken the stands with a heavy stippling brush. Bear in mind, glass in tension is very strong.

No argument doing 200 gsm chopped mat, the brush will not break the strands.

I also accept that 20 gsm cloth is not as strong as nylon.But, why break it with a brush. My experience, examining the wreckage, is, I am surprised just how resilient a 20 gsm finish is as a structural member in an airframe.

[McG 6969]

Don, could you please explain how you can' break' the glass stands just with a 'stippling' action which is supposed to be totally 'vertical' to the surface without bringing any stress to the glass... ?

In fact, using a roller or even a 'best friend'-card - sorry gents - can 'disrupt' the fragile woven tissue.

And, "200g chopped mat"? I believe this is not for RC modelling but for industrial washing machines, bathtubs, shower tubs or swimming pools... not even for full size small aircraft or helis.

Cheers

Chris

[Don Fry]

The strands are so fine in 20 gsm cloth, a brush will fracture them. The rougher the brush, or the brushing, the more fractures you get. Hence the fur you see on the surface when the brush catches the broken strands. And while your brush is held normal to the surface, most of the time, the bristles bend when they hit the surface, pushing at strands.

Hence a roller.

The original poster was using a large brush for boat building, 200 gsm is what they use on large hulls. You are trying to lay down 15 to 20 mm of glass. Not our world.

But my experience of seeing boat building, Sealine, is they seemed to use a lot of rollers to apply resin in the hull construction process.

[Don Fry]

Chris, if you wish to stipple I wish you well, it will do the job well enough. I use a roller. It does the job well enough. But I think neither of us use will think a 2 1/2 inch brush is a tool for our work.