By Phil Clark
Using glass cloth and epoxy as a surface finish on large models is the choice of the majority of large model builders, including every pilot of the USAAF Team. It has many benefits over the more conventional finishing systems, firstly, the large amount of strength is gives the airframe, not only to withstand flight loads, but also the dreaded 'hanger rash'. As we all know to our cost, larger models are harder to move and transport around, and the odd scrape is inevitable. Secondly, the tough smooth surface finish is ideal when we want to represent a smooth metal skinned airplane, the smooth finish will also readily accept all kinds of added surface details, including rivets, panel lines and hatches. Lastly, glass and epoxy should be no heavier than a more traditional fabric and dope finish is carried out properly.
Many articles have been written in the modelling press about 'How to' glass skin an airframe, some good, some bad, some just too complicated! If this is your first attempt at glassing, please get rid of any ideas or horror stories your club mates may have told you, this process really is easy, and not that time consuming once you have an airframe or two behind you.
The key to a good final result is good preparation of the airframe, a clean tidy work place to do the job, and the right product to apply to your model. I'll mention the products to use first and get to the preparation etc. later. If this is your first attempt, I would seriously recommend that you use a cloth/resin system that gives known results. Don't be tempted to go down to your local car repair shop and buy some surface tissue and resin. These are industrial products, and not really suitable for use on lightweight models, believe me. I did this the first time round, and paid the price, having to use a Wolf power sander to remove excess resin and get a usable smooth lightweight finish.
Several resin systems are available through your local model shop or mail order, and can be used on any size of model, but the weight ( in oz ) of the glass cloth varies considerably, and it is this that changes with the size of model. The heavier the cloth, the more resin it uses, the heavier the end result will be. The aim of the game here is to get a durable, smooth, tough finish, with minimum weight, so using a heavier weight cloth on a .40 size sports model is a bad idea!!
Three resin systems that I would recommend are Ripmax SP113, ZAP Z-Poxy finishing resin and Fibretec 'Flow Lotion' All are good systems and fairly equally matched on price. Several makes of cloth are also available from your local shop; the Ripmax brand is probably the most common in the UK, being around about 1/2 oz sq meter, this is perfectly suitable for most large model applications, the Team's B-17 was skinned using this cloth. Fibretec offer a good range of cloths in various weights, but the Fibre 20 and Fibre 50 are the two usable skinning cloths for this application. The 20 is a very lightweight cloth, probably a little too light for large models, but fine for standard club sized models. The 50 is around about 3/4 oz sq meter, this would be my personal choice for skinning models of around 10ft span and over ( my B-26 is done with this cloth ).
The other 'must have' items that you'll need for the job are a good supply of mixing vessels, the plastic measured mixing cups are good, as all resins require careful 'ratio' mixes, a measured container makes this job a lot simpler, avoiding guess work. Latex type gloves, epoxy can be nasty stuff, especially the catalysts, it's best to avoid skin contact. Mixing sticks, short lengths of 1/4 inch square balsa or old paintbrush handles are fine. Paint brushes ( about 1/2 inch wide ) don't bother buying expensive ones, cheep ones from your local DIY store will be fine, epoxy tends to kill brushes, once the model is skinned, the brush will often be thrown away. Cellulose thinners for brush cleaning. Facemask for use when rubbing down, epoxy and glass fibre dust is not good for the lungs. Rubbing down is best performed with wet and dry paper used wet.
As I mentioned earlier, preparation, both of the airframe and in the workshop is the key to a good final result. First off, the airframe MUST be smooth, any small gaps and knocks must be filled, wing skins must be flat and free of steps between sheets etc., If the airframe is not flat, once the model is glassed, you will not be able to sand these imperfections away or you'll go through the glass, the only way to even out the surface will be with filler, this is both time consuming, and can add a lot of unwanted weight.
To prepare the workshop, simple TIDY UP !!, put as much stuff away as possible to leave a large enough area to work in with only the bits and pieces you need to hand. It's a good idea to Hoover your work bench too, any small bits of wood bust/chippings etc., that get under the glass cloth will dry in place and require work to remove them later on.
I'd always recommend you start on some smaller pieces first, tail planes, elevators etc., are a good starting point if this is your first attempt. It is also perfectly acceptable to skin individual components prior to final assembly. Skinning a tail plane and fin can be made a lot easier if the fuselage isn't attached. Attaching them later, leaving yourself just the joining fillets to do is fine, epoxy can be used here to bond everything firmly in place with no fear of it parting company later on in it's life due to a poor joint. Always skin control surfaces prior to gluing the hinges as well, life can get very fiddly if you don't.
Always glass one side at a time, never try and do one side, then wrap the cloth round a tight radius (Leading or trailing edge). Even though the cloth is very thin, once wetted out, it becomes quite stiff, and will not accept tight radii. For instance, when skinning a rudder, do one side at a time, and the epoxy will soak into the wood at the thin trailing edge and harden it, therefore, there is no need to try and get the cloth around the tight trailing edge. For wings and tails, wrap the cloth just over half way around the leading edge and stop, once cured, repeat on the reverse side, once complete, the centre of the leading edge will actually have two layers of cloth on it.
Once you have the airframe finished and your happy with the surface finish, it's a good idea to support the model off the board whilst you apply the cloth. I always try and produce a blue foam cradle for the fuselage; this can also be a useful flying field aid for assembling the model as well. For wings and smaller parts, raise the part of the board using foam blocks, the last thing we want is to damage the model on the board half way through the glassing process. Next comes cloth cutting. Don't try and be too exact here, it's best to leave about 1 inch of excess all the way round the part, this will give you enough to get hold of to pull any creases out when applying the epoxy.
When mixing the resin, don't be tempted to mix one big batch that you think will be enough to complete the whole job ( wing top, side etc. ) This process doesn't use as much resin as you think, chances are you'll end up with some left over which will just go to waste. If you do mix up too much, it can create enough heat to go off in the pot. Mix only small quantities at a time. If you run out part way through, simply mix some more; the area you've already done won't have started to cure as most resins have long working pot life.
As mentioned earlier, epoxy resins require accurate ratio mixes; this must be achieved accurately for the resin to cure correctly. Don't be tempted to add a little more than the recommended amount of catalyst to speed up the cure time, chances are it won't, and the resin will not cure correctly, leaving you with a slightly rubbery mess that's impossible to rub down to a nice smooth finish. You can mix resin by volume ( 50/50, 75/25, depends on the make ), but by weight is the most accurate, if you have a set of digital scales that will measure down to 0.1g, use them. Once measured and thoroughly mixed, the resin will be full of tiny air bubbles, it's a good idea to let the pot stand for a few minutes to let the bubbles rise to the surface and dissipate, this just produces a smoother mix that's better to work with.
The purpose of the first coat of resin is simply to stick the cloth to the airframe, nothing more. With the cloth laid out over the part to be skinned and smoother out by hand, poor a SMALL puddle of resin onto the surface in the centre of the part. The epoxy can be thinned with a little methylated spirits to allow it to flow better. The next bit sounds crazy, but it works a treat. Using an old credit card as a squeegee, scrape the resin out over the part from the centre outwards. Why a credit card, well, it's flexible enough to bend to the contour of the surface it's running over, and it has nice smooth rounded edges that won't dig into the cloth, ideal!!
Keep scraping the resin over the surface until the cloth is wetted out and goes translucent. If you have an area that is glossy, then there is too much resin there ( keep scraping until it's gone ), if an area is slightly white, then it's not wetted enough and more resin is required. What we're looking for is a finish that is smooth and satin in appearance, this is the sign that we have just the right amount of resin to stick the cloth to the model. Glossy means too much resin, too much weight, and a lot more rubbing down later. When it comes to corners etc., it's best to use your 1/2 inch paint brush to apply the resin, and stipple the cloth round the edge in the same sort of way as you would apply tissue and dope. With the part at this stage, leave it alone to cure for at least 24 hours before carrying on to the next stage.
Once the resin is thoroughly cured, the excess over hanging cloth can be trimmed off. I have found the easiest way to do this is using medium grade sand paper to run along the edge of the surface, removing the cloth where the resin finishes. This also serves the purpose of feathering the edge slightly ready for the cloth on the other side to overlap slightly. The glasses surface can also be rubbed back very slightly at this point. Don't try and get it super smooth yet, just knock off any high or rough spots. Use a good quality course paper for this ( 80 grit ) preferably on a sanding block, any flat surface deserves to be block sanded. You will also probably find that when the first coat of resin has cured, the surface feels slightly waxy. This is a common feature of epoxy resins, but it is something that must be removed to enable the second coat of resin to adhere to the first properly, a light overall sanding is a good a way as any for this purpose.
With this done, continue skinning the other side / half / top / bottom until the airframe is completely skinned and rubbed back as described. We are now ready to apply the second coat of resin, the purpose of which is to fill the weave of the cloth, so giving us a surface to rub down to. We want enough resin to rub down to a smooth finish, too much will mean more time spent rubbing down and probably too much weight, too little will mean we rub down into the cloth layer before we get a smooth enough finish. This coat can be applied in two ways, carded on like the first coat, or brushed on. The brushing option if chosen must be done carefully so as not to apply too much resin, remember, fill the weave and no more.
This is the time to coat any exposed areas of un-skinned wood as well, engine bays, wheel wells, fuel tank bays etc.
Down and Finishing
With the resin thoroughly cured, take your 80-grit paper, and remove the shine from the surface, pay particular attention to any uneven overlaps, lump and bumps. Having a single point light source is handy here to pick up on any areas that need work. Once the bulk of the work is done dry, use wet and dry ( used wet ) to get down to the smooth surface, remembering to ware a mask all the time. Warm water with a little washing up liquid in it help as well. Starting with 120 grit, work your way down to about 400 and you'll have a surface good enough to prime. If at this stage you see any areas that need filling, do it now. Good products to use at automotive fillers as they are very fast drying and easy to sand. They may be a little heavy, but in the small quantities we'll be using, it's ideal. For small pinholes etc., acrylic filler putty or 'stopper' is useful to have around, again, available from most automotive shops.
With the whole airframe skinned and rubbed down, a light primer layer is required to highlight any small imperfections in the surface prior to final painting. Automotive aerosols are ideal for this purpose, cheep, fast drying, and rub down really easily. Try and use a high build 'filler' primer, as this will fill the smallest of imperfections. With a light coat over the entire model, rub most of it off!! All we want is for the remaining primer to be sitting in any low spots and small imperfections. Again, use a block on any flat areas wherever possible. Use the 'stopper' to fill all imperfections that the primer reveals.
With the models rubbed down, and all imperfections filled, a
second primer coat can be applied. This one doesn't need to be a 'filler' primer, a normal 'grey' will do fine. This coat can be a
little heavier, as we want to cover all of the exposed glass and the yellow
filler primer, to give a nice even base coat over which to apply the colour
coats. Leave this coat to harden thoroughly over night, as we don't want to
damage soft paint. If you have any areas that are a little rough ( dry over spray etc. ), these can be rubbed back with fine
wet and dry ( 1200 used wet ) or oil free wire wool.
You should now have a completely skinned, smooth, primed airframe ready for detailing and final painting. This process is 'different' to most others, but I think you'll agree, the end results are well worth the extra effort.