From my T-Rex article for CLW:
Finish Affects Design
In the past, I had always designed my airplanes to be assembled from “cores” of either foam or balsa wood fully sheeted in very light contest wood. Traditionally, I never liked “open bay” designs because I liked all of the advantages that came with not having to sand and buff non-sheeted surfaces. Sanding a fully sheeted airplane is like sanding a surfboard, and is very straightforward. In addition to the finishing advantages, nothing infuriated me more than poking a hole through a silkspan covered open bay wing (which I did a lot as a kid). I know you must be thinking that there are tougher market alternatives to silkspan for covering bay wings. This might be true; I just never liked any of them! Silkspan, and to lesser extent tissue, are the only coverings I ever liked finishing with, as I found the “organic” materials to be the easiest to use and the most forgiving. Oh yes, I tried the others, but I always went back to silkspan.
Are you ready for a shock? Recently, I began to embrace the use of film for serious painted finish use. I know, I know, I am a traitor that should be strung up from the nearest tree!!! Film is for the unwashed masses of RC flyers and does not belong on a serious stunt ship. NOT FILM!!!! NOOOO!!! Get a rope!!!
A few years ago, I tried the brand name Ozcover film as a base for painting. Ozcover is heavy clear film that is supposed to be great for painting (uh-no…). I covered my Solace design with this film and applied my standard automotive primer, basecoat, and clearcoat finish directly onto the film. While I missed the silkspan in the finish, hated the spotty adhesion, and some of the poor film qualities of the Ozcover itself, I could see a tremendous advantage to the painted film “idea”.
Shortly thereafter I used film for the finish of my “kit bashed” Brodak Strega ARC into the “Valkyrie”. For the “Valk” I used red Monokote as a base for the wing and tail appendages. I finished and trimmed the film with paint without any regard for the fact that it was film, and it worked great. Many people who have seen the Valkyrie up close have commented that no one would even know there was film under the paint job.
Phil Cartier from the Core House (a supplier of CL combat and stunt stuff and all around great guy and innovator) sent me a sample of his SLC film (Super Light Covering) which is very light clear laminating film etched for adhesion. I found this film to be the “friendliest” and by far the lightest film I have ever used. It, irons on like a dream, shrinks at low and high heat, is very tough despite being very thin and weighing about ¼ as much of most other commercial films, and is dirt cheap (a role of 100’ costs about $25). Phil says that the film is great for direct finishing with paint because the surface of the film has been etched for ink printing, and Phil has done many painted ships this way. I am just too much of a “girly man lightweight” to pull masking tape from film in a multi-layered paint scheme. There is no greater sinking feeling in the world than pulling off a layer of masking tape from your nearly completed stunt plane and seeing PRIMER WHERE PAINT USED TO BE! I want a substrate for my paint jobs that I know has absolutely bulletproof adhesion, as my heart just can’t take it.
To that end, I found an alternative approach to painting directly on the film. I began testing SLC film as a “base” for a traditional silkspan finish. I began applying silkspan over the top of the film with water based polyurethane. In many ways, this appeared to work better than anything I had tried, maybe ever. I prefer using water based polyurethane (like the common brand name Minwax Polycrylic) to apply silkspan rather than using dope. Both my wife and I do not like the smell of nitrate dope and thinners, and my workshop is in the house. Did I mention MY WIFE does not like the smell? I have used Minwax Polycrylic water based polyurethane in the past, and I am currently using water-based polyurethane that was left over from finishing our tile floors that literally has no detectable smell. This is huge in my world, brutha!!! It is also thinner, lighter and much stickier than Polycrylic.
Note: Real men use the “stinkiest” finishing method they can find. Some of the most hardcore real men even change to even more stinky processes as they become available. Real mean love to stink, let’s face it… Nothing makes a real man happier than going to the paint shop and saying, “Hey, you got that new SUPER STINKY paint? I can’t smell the old stuff any more; I think my nasal passages are sealed shut with resin”. I guess I am not a real man any more. I like quiche now. I am a huge quiche eater when it comes to finishing…
Unfortunately, I never liked water based polyurethane (WBPU) for doing open bays because I could never get the hang of shrinking the open bays tight. Because of this, I always stuck to fully sheeted planes. Using the film as a base for the tissue solved this problem. Basically, once the plane is covered in film, there are essentially no open bays that require a tightly shrunk paper covering. Applying silkspan type tissue to a film covered plane is essentially like covering a fully sheeted plane.
Initially, my procedure for applying the paper was the same as the procedure used by “The Master” Bill Wilson. The PU is applied directly from a spray bottle onto the dry paper while hanging from the edge of the work table. Once the paper is wet it is draped over the piece to be finished, and the wrinkles pulled out. The paper is then smoothed with the hands and fingers or a brush. Once the paper had been smoothed, the PU was then flashed off with a hair drier, the edges trimmed, and that is it. Once the paper is dried after sitting overnight it is essentially ready for primer. There is no need to fill the paper any past that point because the water based polyurethane did not sink into the wood or paper.
Recently, I discovered a variation to this method that makes applying the tissue even easier. After using 70% isopropyl to clean my polyurethane brush, I ended up with a milky white jar of isopropyl alcohol. On a lark, I used this mix to apply the paper to a few panels. I found this mix of approximately 10 to 1 of isopropyl alcohol to polyurethane to be the perfect material for applying the paper. In fact, this mixture behaves more like dope than water based polyurethane for the application of the paper. The alcohol (unlike water) dries very, very quickly, is much thinner, has sufficient non-water solvent to keep the paper from getting soggy, shrinks the paper tighter, and has more than sufficient adhesion to apply the paper. By far, this mixture is much less messy, sticky, and does not clog the spray bottle tip, trim blade, and scissors. All this, and the mix still has no stink at all. Once the paper is applied with this alcohol mix all that is required is a full strength coat of polyurethane afterwards to adhere and seal the paper.
One thing to note is that these water-based polyurethanes are by far the most adhesive basecoats I have ever tried. Nearly any kind of solvent based finish seems to stick to it like glue. The reason for this is that they are NOT solvent resistant. The best undercoats for painting actually melt a little when solvents are applied. This is why “non fuel proof” dopes like nitrate are considered a universal basecoat for just about any kind of finish.
Using the SLC film as a base for the traditional silkspan finish solution was literally killing “four birds with one stone”. This finishing method allowed me to:
* Apply my typical silkspan base with either water based PU or nitrate dope
* Fix the puncture resistance issues on the open bayed areas
* Completely seal the airplane so that the finish cannot sink into the wood which causes extra weight
* Cover the surface of the balsa in a smooth surface that does not require any fill at all.
These are all good things!!!
In terms of paint compatibility, once the airplane is covered in tissue, the paint used in the finish does not “know” the plane was ever covered in film. The silkspan with PU (or nitrate dope if that is your thing and you are not happily married) sticks to the film like concrete, and the user can still layer and fix the base using the silkspan. I like to use layers of silkspan paper for transitions, seams, divots, etc.
Topic: Covering SLC (Read 9448 times)
