Tips on Glassing

Overview Using glass cloth and epoxy resin as a surface finish on foam or balsa models is the choice of most model builders. 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. 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 about 'How to' glass 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 friends may have told you, this process is really easy and you can see it in the following attached pictures. It’s not that time consuming once you have an airframe or two behind you. The key to a good final result is good surface preparation of the airframe, a clean tidy work place to do the job, and the right product to apply to your model. If this is your first attempt, I would seriously recommend that you use a cloth/resin system that gives known results. . Several resin systems are available through our local model shop 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! I would reckon to use the lightest cloth like ¾ oz for foam plane as weight is the major issue after all.   Resin and things you need Resin systems that I would recommend will be the Z-Poxy finishing resin which you can get them easily from our LHS. 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 will be used for mixing the resin, cheep ones from your local DIY store will be fine. Facemask for use when sanding down, epoxy and glass fibre dust is not good for the lungs.  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 with either foam or balsa filler. 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. Getting Started 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. 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.   Applying the resin 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, pour 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. Many modellers use old credit cards as a squeegee but I would reckon that you use a poker card instead coz it’s more flexible and better on curves and corners. Once done, leave it to cure for at least 24 hours before carrying on to the next stage. Second coat If you like to have a better finishing, perhaps you can go for a 2nd coat. To keep the air frame light, normally I just go for one coat. 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 . Sanding Down and Finishing With the resin thoroughly cured, use 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. Next 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. Once done, here you go…… You are ready for your paint job.