Covering Guide

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Graham
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Covering Guide

Postby Graham » Wed Oct 28, 2015 9:00 pm

Model covering using our self adhesive heat shrink materials

Dope

The covering materials available to us today are generally superior and much easier to apply than the original silk or tissue mediums. Not so easy to source these days but still popular amongst vintage modellers is lightweight nylon. Although very strong this is mostly used with cellulose shrinking dope, a brushable clear liquid which has a very strong odour generally found to be most unpopular with the ladies. The smell tends to linger or even drift around for hours and has a tendency to cause light headedness if not used sensibly in a very well ventilated area. Coloured and white tissue papers also for use with shrinking dope are still available in both light and heavyweights from specialist suppliers. For specialist finishes there’s a 2 pack liquid resin/hardener for applying thin ultra light “glass cloth” to model structures. This superior finishing medium is mostly intended for large scale and professional users, but also finds favour with seasoned modellers.

Quick fix sticky tape

Finishing and covering models is a task many modellers either hate doing or expect to achieve finished results in short time. At the outset, let’s get that one out of the way, there is no real quick fix to a good standard of finish. There are quick fix coloured sticky tapes on rolls which are best suited to EPP or similar flexible plastic foam products. They may however be useful for adding colourful trimming patterned schemes to enhance the overall visibility of a model. These are similar to brightly coloured parcel packaging tapes. As these are cold fix “coverings”, they are easy to apply and offer a simple field solution to repair minor damage.

Material choice

There are many types and grades of “iron on heat shrinkable” coverings available for us to complete those traditional all balsa airframes. These coverings are intended to provide a tough plastic finish which is fuel proof, water proof and very durable. Some coverings may also be applied directly to rigid foam materials found in many ARTF models. Traditional hot wire cut polystyrene veneered components such as wing panels are particularly suited to iron on coverings. The best type of covering material will often be dictated by the individual model type and weight. Ultra light covering materials are really best suited to indoor flying aircraft and although such coverings are extremely thin, they do provide a reasonably strong puncture resistant outer ‘skin’ which can add overall structural rigidity to a model.

Adhesive layer

Heavy duty textile, or fabric iron on coverings are available in many colours for model aircraft. These are very similar to those used for full size self build aircraft projects and are best suited to larger I/C powered models or as a modern replacement for nylon and shrinking dope as they tend to be much heavier. The textile coverings come in 2 basic types, one has a built in shiny fuel proof external finishing barrier, the other is a relatively matt sheen finish which will quickly soil if not protected by some kind of sprayed or painted on lacquer. Most of the coverings available have a heat sensitive adhesive layer which responds to the heat from the covering iron. This provides a very strong adhesive bond between covering and airframe. Temperature is important but not hyper critical and is often a trial and error learning curve utilising a few off-cuts. Too much heat will simply wrinkle and destroy the material instantly, whilst insufficient heat will fail to provide an adhesive bond. It’s a simple matter to create those off-cuts by first of all cutting a roll of covering into panels large enough to cover the model and use any left over scraps.

No steam irons please

Some modern lightweight coverings have no adhesive backing and require a base coat of special PVA based adhesive to be painted onto the airframe prior to applying the covering. Nearly all of these materials are applied and shrunk taut using a medium hot iron. Specific covering irons are available which have fully adjustable thermostatic temperature control. A small domestic cloths iron can be pressed into service to great effect as a cheaper alternative. Please don’t even consider trying the wife’s best steam iron. A lightweight none steam iron should be obtainable from many electrical outlets or even a market. Those inexpensive compact “travel irons” would also work well if variable temperature settings are provided. A heavy appliance is not recommended as this will leave marks in the finished surface. Before switching on the appliance, always reduce the thermostat to minimum as most coverings will melt if too high a temperature is applied. Steadily increase the temperature until the adhesive backing responds and provides an adequate bond. The bottom of the iron can be cleaned cold with thinners, or heated and rubbed vigorously on a scrap cloth or old towel to remove any coloured adhesive residue.

See through or solid?

Polyester coverings are very strong, generally easy to apply, and have a shiny external finish. Some materials are multilayer where the colour coating is locked away within. There are some metallic finishes and even patterned types available. A range of “see through” coloured coverings are available and these are particularly useful in showing off the internal build quality of built up wings and airframes. These look particularly good on gliders, electric sport and vintages models. Metallic coverings should be avoided where 2.4GHZ radio systems are in use due to signal absorption or reflection. The coverings which have solid colours are generally very dense and may be used to hide a multiplicity of building sins. Most gripes with this type of covering are related to getting it to wrap neatly around wing tips, radius’s or awkward compound curves without wrinkling. There’s no quick fix to this problem other than patience, practice, and experience. Some practice on a few bits of scrap building material is advisable before contemplating covering a new project for the first time.

Fillers

There are some critical points to remember regarding use of heat-shrinkable coverings and these largely depend on the type of surface the covering is being applied to. For instance, any open framed traditionally built up structure will distort and warp if the material is incorrectly applied and inappropriate shrinking techniques are used. All balsa airframes must be dusted down first, and small indents are best filled with lightweight water based filler. You can pay around £8 for a small tub of “special filler” from the model shop or use something equally as good available in large plastic pots from one of the numerous £1 shop outlets. This is a lightweight water based general purpose filler intended for preparation of woodwork or plaster before decorating. They sand very easily and are every bit as good as the “special model filler”. Adding a small amount of water to this type of filler allows it to be easily worked and an old credit card is ideal to apply it. It would be fair to suggest that not everything we are offered as special for modelling is in fact special at all. Polyester resin based car body fillers should not be used as they are difficult to sand and are heavy. The ideal filler needs to be almost as soft as the balsa wood surrounding it. A simple rule applies here, and that is the better the finish on the airframe, the better the finished visual impact.

Tools

Rushing a covering job can lead to disappointing results, whereas a correctly covered model generally enjoys a longer lifespan (unplanned arrivals back to terra firma excluded). Carefully applied covering material not only looks good, but adds a certain amount of structural rigidity to the model. No special or expensive tools are required to make an excellent job of covering your latest project. Everyone will have their own favourite type of knife for modelling purposes, but an ultra sharp surgical scalpel is the ideal tool for neatly trimming the covering material when fixed to the airframe, spare blades in different shapes are available. As these plastic covering materials are quite hard, the knife blade will be become quickly dulled. Rather than dispose of the blades, an economical answer is to simply re-sharpen them using a small oil stone. This can be done a few times before the blade is considered to be scrap. For a superior finish the covering material can be carefully shrunk all over using a modelling heat gun or even a small paint-stripper heat gun with low heat setting.

Bench rash

Let’s assume we have a pristine airframe fully sanded and dust free by either gently brushing the surfaces or carefully vacuuming. A suitable colour scheme will have been already decided upon, tools such as scissors, scalpel and covering iron all to hand. There’s one more thing to do before starting the covering job and that is to thoroughly vacuum clean the whole bench and working area. Failure to do this results in bits of bench debris getting stuck under the new covering, or even worse nasty dents and dings appear in sheeted surfaces....it’s called “bench rash” and looks terrible on a finished model. This problem can be easily overcome by investing about 2 quid to buy a cheap foam camping roll from many high street stores. These soft sponge sheets are roughly 2 feet wide by 6 feet long….a perfect anti rash mat! The sponge sheet is also ideal for packing receivers, fuel tanks and receiver battery packs in that new model, or even when transporting it in the car.

Protective layer

It’s time to get sticking that shiny new covering onto a model. Panels for wing covering need to be cut with a 25 to 30mm overhang margin all round to allow the material to be gripped. The covering available from Giantshark is really good quality and seems comparable to many other commercial covering films costing roughly twice as much. Please note that the heat sensitive adhesive layer is protected by a very thin polythene peel off layer. This protective layer must be removed before use. Having cut the material to size, only remove the backing from one panel at a time or as required. If you find it difficult to find a point at an edge of the backing to start peeling, try using a sharp pin to lift a corner. Rather than snatching at this backing, just peel it off slowly in a linear fashion. Once removed you may notice a matrix of tiny adhesive dots covering the entire surface. Start on the wing underside and carefully tack the covering at the centre section, then a point at the tip centre having gently pulled the material taut. Continue tacking at approximately 100mm intervals and gently pulling the covering tight all around. (Not over tight…remember this is HEAT SHRINKABLE). Carefully run the covering iron around the entire outside edge, taking extra care at the tip to ensure as few wrinkles as possible. Grip the overhang and pull the material whilst applying heat. Now run the iron over the whole seam to provide an overlap of 5 to 10mm on the top surface.

Slack

When a satisfactory seam has been completed, any surplus material is now trimmed off.

This trimmed seam is now once again sealed down onto the balsa surface. Don’t worry about the slack appearance of the covering at this point. Repeat this procedure on the other wing panel underside and trim off as above. Where the covering overlaps at the centre section, iron down to ensure the top covering sticks to the bottom covering. Larger areas can be heated with the iron then followed up by quickly wiping over with a duster whilst applying moderate pressure to move any air bubbles. Any stubborn or remaining minor bubbles can be pricked with a pin and ironed down again.

Topside

With the underside completed, add both top side panels in the same way. By this time it will almost certainly be getting much easier as confidence builds and personal techniques and preferences are used. There is nothing better than experience for this job. With all of the trimming completed it’s a good idea to visibly double check for any loose edges which may have escaped the iron. Any unfixed edges which are not effectively adhered will almost certainly result in the covering lifting or tearing in flight. The whole wing area can now be shrunk to a drum tight shiny finish by carefully and slowly applying heat either from the covering iron or a heat gun set to low temperature. Getting too close with a heat gun will result in a melted hole in your nice new covering. Be patient and work from the centre out and turn the wing over to complete the same amount of shrunk area top and bottom. Even shrinking top and bottom should provide a warp free surface. Re-heating is possible should minor warps be noted. Carefully twisting an affected panel whilst the covering is being gently heated should remove any minor warps.

Trim

Once completed, additional self adhesive coloured trim can be added as required. This is best applied onto a really dust free surface otherwise specks of debris do tend to collect and become a permanent ugly feature. Self adhesive trim is available from model shops or you may want to try your luck by asking the local commercial vehicle sign man for some thin off-cuts of material. These are mainly vinyl based, very flexible, easy to cut and have a super sticky backing. Most often these will go in the bin and you may just get lucky with some free scraps. It’s a good idea if possible to cover items such as rudder, fin, elevators, ailerons, tail-plane before the model is fully assembled. Do remember to remove sufficient covering material where any gluing or fixing is going to be done.

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