ArticlesBe a film star

(By Chris Walker)
Achieve results like this!
The above model is a scaled down version of the BSD Thor (Modi - son of Thor); by Chris Walker - decals and covering finished in HeatShrink

The film I'm referring to in this case is, of course, heatshrink film; as used by thousands of aeromodellers the world over to achieve an attractive, durable finish.
I was an aeromodeller before taking up rocketry, and when I started building rockets I wondered if the stuff could be used on rocket tubes and fins whilst staying put.

Colours and types

There's an almost endless variation of colours from standard through to wickedly fluorescent, you can even buy checker-board patterns and carbon-fibre effect. With the exception of this special pattern stuff, heatshrink film is cheap to buy from model shops and £10 worth will see you through several average LPR models. The two main brands on the market are Solarfilm and Profilm, and each has its enthusiasts. My experience is mainly with Solarfilm. I'm told you can use higher temperatures when working with Profilm, so perhaps it might be even more durable in rocket applications? So far(Oct 03) I've only used film on LPR rockets, but the film seems to be hanging on grimly through over 50 launches of various rockets(up to D power), so much of the info which follows could perhaps be tried on HPR.

Trial runs

I very strongly suggest you do some trial pieces on old body tubes, loo rolls and some pieces of balsa or ply (to simulate fin covering) before trying this for real on your rocket. It will also enable you to establish the correct "tacking" and "shrinking" settings for your heat iron. Be warned; some makes of film can be all but impossible to remove cleanly if you make a mess of it. They can leave behind very sticky, coloured residue.

Be patient

Make no mistake: film IS NOT a quick route to a finish. It might, in some cases, be quicker than paint, but putting film on well requires patience and care. Some of the main advantages are that it's not smelly or messy, and can be applied in the warmth and comfort of your home, instead of in the inhospitable shed or garage! Also, in general, film isn't too happy on non-porous surfaces. It seems to go on fine over standard paper/card body tubes, and even over fins which have been coated with cyano (my usual wood-finishing technique). On non-porous surfaces there's a great risk of major air bubbles under the surface. However, I'll hopefully be doing some experiments soon on glassfibre-finished tubes to see if it can be done.

Splash the cash

I would advise investing in some tools. Take a trip to your local model shop and for a relatively modest outlay you can get a film iron, and - especially if you're planning on doing a big rocket - a modeller's heat gun. Domestic irons can be used, but the weight of them don't half make your arm ache, and hairdryers just don't get hot enough. Paint-stripping heat guns are too fierce. Alternatively, you could always borrow the gear from your local friendly aeromodelling buddy.
OK, the tools cost money - but how many expensive paint spray cans do you get through per rocket?
Also, buy a small bottle of stuff called Balsaloc. This is like an extra adhesive to the heat-sensitive stuff already on the film, and is really handy for making sure the film is stuck in critical areas, such as the mouth of body tubes where you won't be wanting to wrap the stuff over, or on concave curves, like fin fillets. You apply Balsaloc to the part, not to the film itself.

Planning

Before you start; have a plan! For instance, glue doesn't stick to film, so you need to be thinking about which bits you're going to have to stick to the rocket (e.g. the fins) before you start filming, and which bits would make it awkward if you did so (e.g. tube couplers).

Preparation

As any bloke who's avoided the decorating for months on end will tell you, the secret is in the preparation!
So you've got your completed (or as completed as you need it) rocket in front of you. Let.s prepare to film it.
Make sure any area you.re about to film is dust-free. Have to hand a scalpel with a fresh blade, sharp - preferably pointed - scissors and a steel straight-edge, plus a pin (more on this later!).
Using a piece of plastic foam or a small brush, put Balsaloc (see instructions on the bottle) on all the edges where the film may simply have to be stuck and not wrapped over(e.g. mouth of body tube), and on any concave areas(fillets). Another area might be around the launch lug. In the case of rail buttons, I'd suggest putting them on after the rocket is finished, by removing/scraping away a small area of the film to give a glueing area. Removing larger areas of film isn't recommended. As a guide, on an LPR rocket I'd put about a half inch band of Balsaloc around the mouth of a tube, and about a quarter inch around the launch lug, and also over the fillets and just either side of them.
Balsaloc dries very quickly, usually in about 15 mins. Don't put it on too thickly or it will show underneath the film.

Covering

Dealing firstly with the fins. These should be pre-finished using your normal method. Either simply sand smooth before filming, or use a coat of cyano glue and sand back to smooth finish before filming. I can't vouch for whether sanding sealer works in the same way underneath film, but I reckon it should. A point here is that standard film adds little or no strength to any assembly, so unless you've got something like a ply core/balsa laminate construction, I would recommend using cyano or sanding sealer to strengthen the fins.
Switch on your iron and adjust to a heat whereby if you drop a small piece of film on the sole, the film wrinkles reasonably well. If you buy film in packets, as opposed to off the roll, the packet sometimes carries an illustration of this and you trial runs should help here.
Cut a piece of film oversize(about half inch to inch overlap should be right) but with one nice straight edge which will lie along the fin root/fillet. Now, using the aforementioned pin, flip the film piece over and "winkle" the pin at a shallow angle at the edge until the point scrapes a piece of the backing free - and then remove the clear film backing piece. Keeping the coloured film reasonably flat against the fin, with the straight-cut edge slightly overlapping the fillet, lightly tack the film to the fin in a few places around the edge. If you get a really bad wrinkle, gently lift and replace. Then, move the iron all around the edges to seal them down. It helps to avoid wrinkles if your movement of the iron is away from the main part of the piece towards the edge whilst carrying out this process. Only tack the edges, not the middle; Don't worry if you've got a few wrinkles at this stage. Pull the overlap over the leading edge and use scissors or scalpel to trim back the film to leave a very small overlap which will eventually be ironed down on the opposite side of the fin. Repeat for the trailing edge. At the fin tip, you'll quickly figure out how to snip the film and trim it to achieve the same effect - the scissors can be very useful at this point. Using a slightly rolling motion with the iron, roll the overlap round onto the far side of the fin(about an eighth inch overlap should be sufficient. I suggest you do the leading/trailing edges first and then the top.

Now shrink it

Turn the iron up a notch or two to what you've established as "shrinking setting" during your trial runs. When it's risen to temperature, glide it over the film, just a millimetre or two above, working from the centre outwards, and you'll see the film shrink. Keep doing this until you seem to have shrunk all of it. Where there are stubborn wrinkles, be patient and keep the above process going ' you'll be surprised just how much wrinkled film will straighten eventually.
On very large areas, a heat gun could perhaps be used, but be sure not to apply heat in one place for more than a few seconds. Heat guns have the capacity to melt the film if used over-zealously! An iron alone should be sufficient for most LPR applications.
Check the film for air bubbles and very lightly prick any with a pin - or you could do as I do, and prick the general area a few times in any case as well. Glide the iron over the area again, in sections, and as each section heats up, rub it onto the surface of the fin with a cloth - this adheres it fully to the fin surface and the air bubbles will disappear. Run the iron over the edges to make sure none have lifted slightly during the shrinking process, and to firmly seal them down. Repeat for the other side of the fin - and now you only have two or three more to do!!

The other bits..

Next, I'd in-fill with film on the body tube between the fins, using virtually the same process(with the exception, of course, of cyano/sanding sealer finishing), with the new pieces just overlapping the film already on the fins. You might have to custom cut the front of these pieces so they lie nicely around the base of the leading edge of the fin. These pieces finish just in front of the leading edges. The main body tube film piece, when fitted, will just overlap them. You will see the "edges" of your pieces when using film, but if cut neatly, you won't be able to discern them from more than two or three feet away. Alternatively, you can hide some of them with film trim later on.

Tubes

For body tubes, follow much of the above process(with the exception, course, of cyano/sanding sealer finishing).
However, you'll have to be more accurate with cutting the piece initially as there's little chance to trim it. I would suggest a piece that's about an eighth of an inch or quarter inch bigger than the body tube diameter.
Put a small cut-out in one edge of the film where the launch lug will lie. This then gives you a start point for wrapping the film round the tube. Tack and then secure the film down along the length of the tube, and carefully use a scalpel to trim as necessary around the launch lug where the film comes round to meet the small cut-out you made to start with. Work the iron around the tube top and around the area just in front of the fins. Trim the top flush very carefully using a scalpel after it's been tacked and secured. The "shrink/pin/heat/rub with cloth/re-iron edges" routine should then be followed. If you reckon your body tube is very flimsy, try putting a coupler just inside the mouth of the tube before you shrink the film, and this should stop tube distortion.

A few hints

You can, of course, further beautify your fabulously-covered rocket with any amount of the self-adhesive Solartrim or Pro-trim colours on the market. There are some special techniques for applying this - but that's another article! In any case, basic application of trim shouldn't be beyond the ken of those who've built a few Blue Peter projects with sticky-backed plastic.!

Run a warm(below tacking temperature) iron over the trim after application, to make it extra-secure.
Even lettering can be cut out of trim using a scalpel and a pattern run off from your computer. However, I find that it's easier to use film - which is thinner and easier to cut - for lettering. Then show the adhesive side the merest hint of cellulose thinners - little more than the fumes will do! This softens the adhesive slightly, and apply the lettering. Finish off by running over it with a warm(below tacking temperature) iron.

One last very important tip; keep an eye on your film edges to make sure they're not lifting after each flight. Iron them down if they are. You don't want your rocket shedding it's covering like a peeling banana as it lifts off the pad!

That's about as much advice as I can offer in a single article in terms of describing the technique. I would stress again that you should do quite a few test pieces, simulating coverage of fins and body tubes, before trying it on your latest prized project! You'll develop your own ways of getting round some of the problems you'll encounter along the way, and see just how far you can "push" the material with the iron.

Process summary:

  • Summon your patience!
  • Roughly cut film to size
  • Remove backing
  • Tack in place
  • Shrink
  • Remove any air bubbles
  • Re-heat and rub down to adhere
Chris Walker was a member of the Wirral Rocketry Association, BMFA and aeromodelling clubs, producing some of the most beautiful models around. In previous lives, Chris Walker Painted for Louis IV, pioneered rockets which were fired against the US under the alias Sir William Congreve, and painted Spitfires in WW2.

Chris passed away in December 2004 and was an astounding man, friend and modeller. He is sorely missed, but remembered with fondness for what he brought to the community.