Like with most finish jobs, most of the work is in the preparation. It's no different covering models with film. For sanding the surfaces you'll need a few grades of sandpaper/wet & dry, some light weight filler, a clean soft surface to place your components on and some sanding blocks and last but not least, a tac rag.
Clean, soft surface - For this I used an old bed sheet folded. Its purpose is to protect the flying surface from dings.
Sandpaper/wet & dry - Start with coarser grade and work down to fine. I started at 240 grit sandpaper then 400, 600, 800 and finish with 1200 grit.
Sanding Blocks - You can get these from any decent auto parts store. The ones I bought are made by AP. I bought the standard one (stiffer) and a soft one for going around corners or curves.
Light weight filler - You could use the old talcum powder and dope here but I had some light weight balsa filler on hand. It works great and weighs next to nothing. I believe it's distributed by Hobbyco and is called Hobby Lite.
Tac Rag - These can be bought from most Auto shops.
Now, there are a few ways to skin this cat (for want of a better term).
- Some use dope or balsa loc/rite on the surface. Can be hard to get because it's flammable.
- Some use "el-cheapo" hairspray sprayed onto the sheeting.
- Not many would use methylated spirits between sandpaper grades. This is more of a cleaning operation but also makes the balsa grain stand up. More on that later.
I did some testing for options 2 and 3. I found that option 3 provided the best adhesion of the film to the balsa. Therefore, for this build, I will be using method 3.
At this stage all your surfaces should be roughly sanded to shape with any hanger rash filled with light weight filler. That is, all the leading and trailing edges, tips and the roots. Grab your sanding blocks and start with 240 grit paper and go over the whole surface gently. Dust off the surface with a soft brush or carefully blow it off with compressed air. Then grab some methylated spirits and a clean rag. Liberaly douse the rag in metho and wipe down the whole stab panel and elevator. Set aside to dry. It should only take five to ten minutes to dry out.
Once the metho has dried off, them move (up in number) down in grit to 400 grit paper. Again, a light sand over the whole surface, dust off and then wipe down with tthe metho soaked rag and set aside to dry.
This is like "De Ja Vu". ;-) With the surface all dry, we're ready to move to the next grade of paper which in my case was 600 grit wet and dry. Sand the whole surface again, dust off and wipe down with the rag doused in metho again. Set aside to dry.
Now move to 800 grit wet and dry and sand the whole surface again. At these finer grades we're not removing much material at all. We are however flattening the surface which in effect increases the surface area which surprise, surprise increases the surface area for the film to stick to. So, once sanded, dust things off again and grab that metho soaked rag again for the last time and wipe the surface down. Set aside to dry.
The last grade is 1200 grit. Sand the whole surface and then dust it off. The panel should be almost glowing now. Final cleaning will be done with a tack rag but we'll do that just before applying the film. Set the panel aside in a safe place where it's mot going to get dinged!
You'll want to have a good think about the design of your scheme. To some extent, the scheme painted on the fuse will dictate what design and colours you use. The bottom side of the surfaces allows a little more freedom as we need to have a good level of contrast between the top and bottom of the model. The top surface design I settled on is loosely based on the factory scheme and the bottom side is just a simple checkered pattern.
Once your design is settled, you'll need to cut out some film. Whist you could do this freehand, a better more consistent way is to use templates. An alternate way is to cut the film in a vinyl cutter which is potentially a more direct way of cutting accurate parts but it has its own drawbacks. The other method is to make some templates to cut the film. These templates can be either cardboard or plastic. I was initially going to make cardboard templates with the assistance of a French curve to get the shapes right. But, I thought why not draw the scheme up in Fusion 360 and then cut the templates on my CNC router out of plastic? Perfect idea! The first thing to do was measure up the stab to draw a profile of it. Then sketch out on that profile the individual colours. Remember, you need to allow some overlap on each film joint. This is easily done by creating offset lines in Fusion. Around the perimeter of the stab I allowed 25mm of overhang. Where the film is to be wrapped around the tip I allowed 150mm of overhang. This gives you something to hold when stretching the film around the tips. Each colour drawn will eventually become a component. In Fusion 360 you can even change the component colour to look similar to your scheme. This will help you visualise whet the finished product will look like. Once all your components (colours) are drawn up they can then be copied into a new drawing and laid our ready for CAM (Computer Aided Manufacture). I learnt this Fusion trick when making the wing templates.... The pictures below show the basic process. Once the template is cut its simply a matter of breaking it out of the panel and sanding the edges smooth ready for use.
Ok, firstly we'll cut out the film parts using our templates. Make sure your cutting surface is clean. Any dust on your film will get transferred to your surface to be covered.
The templates are simply laid on top of the film and then cut around them with a scalpel. I cut them on a self-healing cutting mat and use surgical scalpels. A new blade helps too. Reverse the templates to cut the film parts for the opposing Stab. I found some of the templates flexed a little (whilst cutting) where they come to a fine point. To alleviate this, a small amount of double sided tape was applied to the template to stick the template to the film backing. For the top surface, you're going to need to cut two sets of film parts for each stab. It will allow enough film for the overlaps at the LE of the elevator and TE of the Stab half.
Right, now all the film parts have been cut. We can start to apply the film. Well, not quite.... Remember that Tac Rag I spoke of earlier? Well we need to use that on the surfaces before covering to remove any last bits of dirt and dust etc. The surfaces just need a very, very light rub with the Tac Rag. You'll be quite surprised what the Rag picks up. The surface is now ready to cover...
There are a few things to note when covering:
- Keep things as clean and dust free as possible. The film sticks better. :-)
- Start with the bottom of the surface first.
- Start from the trailing edge first so that overlaps are facing backwards against the airflow.
- Joins should overlap by minimum 3mm.
- Buy yourself a half decent covering iron. I'm using a Prolux iron which holds its temperature very well.
- Take your time and the job will come out much better.
Iron temperature settings are film dependant. The film I'm using is Hobby King. Quite a few people have been using it with good results and the price is good too. I've found that 100C is good for general covering and I wind the iron up to 120C for stretching the film around the tips. But, try sticking some film on a bit of scrap balsa to determine the best heat setting to activate the film adhesive.
The first item to be covered is the bottom of the elevator. To help guide film part placement, make some light pencil marks on the surface. In my case, the design is pretty simple so a couple of centre lines is all that's needed. Remember, overlaps should always face backwards. I Ironed down the first colour just on the bottom side of the elevator and then overlapped the second colour. We now effectively have one sheet of covering to cover the complete bottom of the elevator only that the edges have not been trimmed and ironed down. We need to think forward with covering. We need to have overlap between the top and bottom covering and also on the ends. We want to wrap the surface up like a Christmas present. On the elevator ends we want overlaps facing backwards too. Starting at the elevator trailing edge, you should be able to measure how much overlap you need and mark that on the film at both ends. You want to go past halfway on the surface edges for overlap when the top surface is covered. Then simply use a steel ruler and scalpel on the flat cutting mat to cut the film. You can then iron this section down. Next do the elevator leading edge (bevel) and iron down. On a long surface, you'll find it easier to start ironing the film down from the middle and work you way outwards. Now carefully mark the film on each end to determine where you're going to cut. Don't be afraid to fold the film over the edge to help work out where the cut should be. One the film is cut, think about the airflow... Iron down the little flap at the trailing edge first, then the bottom surface flap and lastly the leading edge flap. This effectively makes all joins face backwards to the airflow. :-) The bottom of the elevator is now covered!
Repeat the above process for the remainder of the Stab half. The only real thing to watch is the alignment of colours with the elevator. You'll also need to increase the iron temperature to stretch the film around the tips. This is where the excessive overhang around the tips comes in handy for holding it. The tips are the hardest part of the whole covering job in my opinion. In the corners of the elevator cut out you want to stick down a small scrap of film in the corners. This allows for cutting a 45 degree cut in the film. See pictures below for a better explanation. I found it easier to trim the film on the tips after it was stuck down. You have little choice really. I marked a line around the tip where I wanted to cut and then carefully followed the line with the scalpel. Take care not to cut too deep. You only want the film trimmed. ;-) The bottom surface is now covered.
Filming the top surface is somewhat more difficult because the colour scheme is more complex. You will also find that cutting two sets of film parts for the top surface is required too. Although this wastes a bit of film, it makes it much easier to get the overlaps at the elevator LE and Stab half TE right. Again, start with the elevator first and work your way forward. All overlaps must face rearwards to avoid the airflow. A soft pencil to mark where the film parts are positioned is a great help too. You can do a dry fit with the colours prior to ironing them down just to make sure everything fits right and at the same time make some alignment marks. As with the bottom, you want to iron down the complete top elevator surface prior to trimming and ironing down the edges. When trimming the edges, we only want to go to halfway this time. This should provide adequate overlap and a nice finish. Once the elevator is completed, it's time to move onto the Stab half. This is where those alignment marks previously made come into action. Placing the finished elevator next to the Stab half will also help a little with alignment but, the overlaps will make it somewhat hard to align the Stab half film parts. Don't forget the small piece of film to be placed in the corner of the elevator cut out near the tip. Once again, start from the rear and work your way forward with each film part to form one sheet of film. Then turn up the heat on the iron and work the film around the tip. Once this is done (turn down the heat) you can trim the LE/TE and root and iron them down. The tricky part is trimming the film around the tip and trying to cut through just one layer of film. A sharp scalpel blade helps here so don't be afraid to swap the blades out regularly. I buy them off eBay for less than $0.30 each.
With any luck you'll now have one covered stab. Trust me, the second one will turn out better... ;-)