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Sails are now often made from ripstop nylon or polyester, which is a woven material coated on one or both sides with a thin plastic film. A small proportion of the threads, a few per inch in each direction, are thicker than the rest and have the effect of arresting any tear. The material was developed as a substitute for silk for parachutes, in which any rip, for example from a bullet hole, would be catastrophic if it propogated.
Other materials used are normal or high density polythene (including recycled supermarket bags) and mylar film, as sold as gift-wrap. Tyvek, which is a woven and bonded material made from high density polythene fibres, is another modern material often used. (If you can remember 5.25" diskettes, they used to come in an storage sleeve which was often made out of Tyvek.) Traditional kites have been made from paper or silk. Whereas paper is still used, silk has been virtually completely replaced by ripstop, in the West at least.
Painting kites is popular in some circles, and has been used for many years in the East to decorate traditional kites made with natural materials such as cotton or paper. Tyvek takes acryllic paints successfully, but ripstop nylon and polyester are harder. Soft fabric paint can be used on these (as sold for decorating tee-shirts, in the UK under the name Dylon Colour Fun), but you must be patient and apply it in thin layers, heat-curing each before applying the next, otherwise it remains tacky. Colours are less vivid than with some other methods, but a striking result can still be obtained. My chi-rho kite (39k), using Charlie Charlton's 1.2m Rok plan shows what can be done very easily. (The chi-rho and the fish are two very ancient Christian symbols which I have combined in this design.)
Much more successful are spray paints as used for colouring artificial flowers, but the downside is that these are normally based on toluene or xylene, which are highly toxic and can easily kill you even before you realise you've got a problem. Worse still, they are cumulative, which means that what you got away with yesterday could kill you tomorrow. Nevertheless, stunning colours are achievable with this method. Here are some experiences of painting on ripstop from an expert in the field. Others have successfully used dyeing on ripstop. And another useful resource on dying ripstop.
Another option is to use T-shirt transfer paper. You print on it with an inkjet printer and then iron-on the transfer. It's reported to work as well on ripstop nylon as on T-shirts and survives washing and scrubbing, but its long-term durability has yet to be established. The main drawback is that you can only do A4-sized sections at a time. Peter de Jong has written some useful notes on the technique.
Many kite builders use a technique known as "appliqué". To take a simple example, suppose you wanted to decorate your kite with your initial. You would cut out the letter in a colour contrasting with the kite sail, and sew this to the back, making sure you sew all around the letter. You then turn it over and cut away the original sail just inside the line of stitching, exposing the letter. A complete picture or complex design can be built up using multiple colours. The technique is particularly effective when the light is behind the kite since unlike paint, much of the colour comes from transmitted light. Only a moderate amount of skill is required - with a little care, anyone should be able to achieve an acceptable result with a simple but effective design on their first attempt - and the time taken may not be any more than that needed to paint and cure several colours.
However, these are too elastic to allow the fine control needed by modern sports kites, for which high density polythene is used, under trade names Spectra and Dyneema. These are very strong for their weight, indeed, it is the same materials, woven into fabric, which are used for bullet-proof vests! A disadvantage is that they have a low melting point which can easily be exceeded if they are crossed with another flyer's nylon or polyester line. They also need to be sleeved with polyester or nylon where they are knotted otherwise the concentration of stress can cause them to fail at well below their rated strength.
Another area where special lines are used is in fighter kites. Control of these involves allowing the line to run through the hands and pulling it in, again by hand, into a pile at the flyer's feet. Normal flying line should never, ever be allowed to run through the hands as severe burns will result. Special waxed or glazed line is available for fighters which can be safely handled in this way.
Oriental fighter kites are flown in battles using line impregnated with ground glass or some other abrasive, the aim being to cut the opponent's line with your own. Needless to say, such lines are not controlled with bare hands!
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