By Rob Millinship, Aero Graphic
For most of us, the process of deciding on the colour scheme for our homebuilt starts around the time the cheque is posted for the purchase of the plans! In fact for some (the writer included!) to dream of whizzing around in an aeroplane with our own totally personal paint job is one of the main motivating forces behind tackling a home constructed aircraft in the first place. But, unfortunately for many of us throughout the years and months with hacksaw and rivet gun in the workshop, there remains the worrying thought that eventually you will have to bring your artistic talents to bear on your engineering masterpiece. For some strange reason the relatively simple process of applying a paint finish to an aeroplane can become a major psychological hurdle to overcome. As a result of this, some projects are shipped out to the professionals to finish, often adding considerably to the overall cost of the project, whilst others suffer with a less than ideal finish in terms of either weight, durability or other quality defects that can spoil an otherwise well constructed aeroplane. The object of this chapter is not, therefore, to tell you what colour and trim to apply to your aircraft. That is your choice and will ultimately reflect your personality and requirements in the finished aeroplane. The object of this article is to provide you with some basic information about paint finishing so that, hopefully, you will be encouraged to tackle the final finishing stage of your project.
First of all it has to be said that with so many different methods of constructing a homebuilt aircraft (i.e. composite, all metal, all wood, mixed wood and tube etc.), there can be no hard and fast rules that will cover all types. However, the basic flexibility of the outer surface of the aeroplane will tend to influence the type of paint system that can successfully be used. For example, there is little point in applying a stove enameled surface to the unsupported fabric surface of a Skybolt wing; firstly it would be heavy, secondly it would be brittle and would crack and flake as the fabric moved and finally, the wings would probably catch fire in the stoving oven! But the same stove enameled finish on the welded steel tube truss of that same Skybolt would be just about perfect, displaying all the right characteristics of hardness, corrosion proofing and resistance to abrasion.
What about equipment? By the time the project has reached the finishing stage, even the simplest of homebuilts will have accumulated a very considerable investment of time and money. For many homebuilders, the thought of purchasing yet more specialized equipment to be used only once will not go down well. However, in my experience it does not make good economic sense to hire spray equipment for aircraft work. Unlike painting a car, or the garden fence for that matter, aircraft normally take considerably more time to paint. This is due to the following problems:
- Homebuilt aircraft are normally sprayed in component form, i.e. wings, fuselage, tail surfaces, cowls, fairings, etc. (Unlike cars and fences!).
- Homebuilders workshops are not normally large enough to lay out all components for spraying at the same time.
- The surface area of even the smallest aircraft is many times larger than a car. Wings and fuselages need paint on both sides - not many people paint the underside of the family car!
- The rubbing down operations between coats can take a considerable amount of time, particularly on delicate fabrics, again due to the large surface areas..
- Most if not all aircraft have, at minimum, a two colour paint scheme.
- Aircraft are generally difficult and time consuming to mask, both initially and between colours. Initially masking is required to stop paint going where you don't want it, i.e. engine bays, cockpit interiors, undercarriage oleo’s, etc. Between colours, masking straight trim lines on a curved surface fuselage can be very time consuming!
The answer is to purchase spraying equipment, which need not be expensive. When the project is complete, sell it, preferably through the pages of ‘Popular Flying’ to the next PFA’er. The amount lost in second hand value will be substantially less than hiring a compressor, gun and mask for a month or so!
As for what equipment to buy, I would suggest a visit to the local vehicle refinishing supplier. Discuss your requirements with them. Don’t be tempted to spend enormous amounts of money on the promise that a particular air gun will achieve incredible results. Top of the range guns are a bit like top of the range cameras - their qualities can only be exploited by the top professionals!
As a bare minimum you will require a reasonably sized compressor. Buy one with a large reservoir if you can, which will deliver a minimum of 6 c.f.m (cubic feet per minute) free air. If you intend to spray two pack paints (more of which later) then you will need a larger compressor (14 c.f.m minimum) in order to provide air for both the gun and your air-fed helmet. Alternatively, two smaller compressors, one for the gun and one for the mask, often works out cheaper and are often easier to sell at the end of the project. Check what is more appropriate with your supplier.
Secondly, you will require an appropriate spray gun. I prefer gravity fed guns where the paint reservoir is on top of the gun. This is opposed to syphon fed guns where the paint reservoir hangs below the gun. The advantages of gravity fed guns are as follows:
- They tend to be mechanically simpler and tend to require less maintenance.
- Every last drop of paint in the reservoir can be used up (important when using expensive two pack paints!). Very small quantities of paint can be be mixed and used, unlike syphon guns which require sufficient paint to cover the tube in the bottom of the reservoir.
- They are easier to clean thoroughly - important if you are swapping between colours regularly. A blob of red paint from a previous mix can reduce even hardened professional sprayers to tears!
- Finally, they are generally cheaper than the equivalent syphon gun - Hooray!
Lastly, you will require appropriate breathing equipment for the type of paint you are going to spray. THIS IS IMPORTANT! Some paint systems, particularly two-pack systems using isocyanate hardeners and also chromate etch primers are very poisonous. It all sounds horribly complex and expensive but in fact the reverse is true. A suitable helmet and belt mounted regulator / filter will be available from either your paint supplier or compressor supplier and will be the best investment on the entire aeroplane. And remember, you can always recover some of the outlay through classified ad’s in ‘Popular Flying’.
Before discussing the actual paint systems themselves, just a short word about the proposed spraying environment. If like the rest of us your workshop/garage has also to double as a spray booth, just a few words of advice. Don’t assume that all of the paint in the gun is going to finish up where you aim it. At least 40% of the paint you will purchase will finish up on items other than the aeroplane! Things like the lawn mower (mine’s still yellow after twelve years!) and all your tools that you forgot to put away. At least you will know which tools you have because the top of your bench will have a perfect imprint of each one sprayed on its surface! This is no joke - modern paint systems have exceptional bonding qualities and in some cases are totally solvent resistant when cured. Not funny if the overspray is on top of your car, or even worse, the top of someone else’s aeroplane! The solution is simple. Build a polythene tent inside your workshop. One roll of cheap polythene from a builders merchant, a roll of sticky tape (I use carpet joining tape - very sticky) and a few drawing pins will create a very acceptable booth around and under your project. When the job is done, the whole lot can be pulled down and thrown away, dust, masking, the lot. Another advantage of making a booth in polythene is that most of the dust generated by the spraying and rubbing down tends to be attracted to the polythene sheet, rather than to the spray job.
So finally, having bought the equipment and built the booth, what about the paint? The right choice of paint system seems to be the main stumbling block for most people. Obviously, it would be impossible within the confines of this chapter to discuss all of the many systems available and appropriate for aircraft use. Therefore I will try to give some of the pro’s and con’s of the most commonly used types. As to which system to use, first seek the advice of the kit manufacturer or the plans supplier or fellow PFA members who have already reached the painting stage. Their experiences, good or bad, will save hours of investigation and may well prevent the purchase of inappropriate paint materials. Some light aircraft are extremely critical on weight, others not so much so. Therefore a kit manufacturer’s advice could be vital, particularly if you are contemplating using a two pack (paint plus hardener) system where the weight build-up can be considerable. For the purpose of this article let us consider an appropriate paint system for a conventionally constructed light aeroplane. That is, a wood, or steel tube structure with fabric covering over an open framework, fabric over plywood leading edges etc. and mixed alloy and glass-fibre reinforced plastic (G.R.P.) cowls and fairings.
Traditionally, cotton or linen fabric would have been treated with an air-drying nitrate cellulose dope, either clear or slightly pigmented for air proofing and tautening. This would have been followed by a mix of non-taughtening nitrate cellulose dopes and aluminium powder to provide an ultra-violet light resisting layer to protect the natural fibres of the fabric from rapid deterioration. The final decorative coats would again be colour pigmented nitrate cellulose dope to provide a durable colour finish. Aluminium components would have to be treated with an acid etch primer as a key for the decorative top coat, again normally a coloured nitrate cellulose dope. This basic paint system, although dating back to the First World War, is still in common use and still gives a light durable finish. However, as with most things, there are some advantages and a few disadvantages with the traditional system. Firstly the advantages. Nitrate dopes adhere well to natural fabrics (linen and cotton) and also give the best adhesion results on modern synthetic fabrics (Polyester and Dacron), although on synthetic fabrics it relies on a ‘mechanical bond’ by surrounding each fibre with dope. They are easy to spray for the final colour coats or to brush for the early tautening and airproofing coats. They are light in weight and easy to rub down and polish. That’s the good news. Some of the disadvantages are that, firstly nitrate dopes will go brittle with age, stress cracks appearing wherever the fabric is subjected to flexing, for instance along the edges of ribs and stringers. This in turn can lead to rapid deterioration due to ingress of water and dirt. This is a particular problem on natural fabrics. Take a look at an old Auster or Tiger Moth to see the problem. This brittleness also creates problems when used over metal panels. Although it will adhere adequately to appropriately etch primed and prepared alloy panels, its impact resistance when applied to comparatively soft metal panels is poor, stone chips and scratches tending to appear quite rapidly. Finally, and in some ways most importantly, it is very flammable even when fully cured.
So what are the alternatives to the traditional system? Firstly there is butyrate dope. Slightly rubbery in consistency, it remains flexible and is not nearly as flammable as nitrate. It should not be used directly onto base fabric as it’s adhesion properties are not as good as nitrate. But for subsequent coats up to and including the aluminium dope layer, it is ideal, being easy to brush or spray. It is also available in colours for finishing coats and can be polished to a high gloss. The disadvantages are that although similar in weight to nitrate dopes, it does not seem to adhere as well to metal surfaces. Most manufacturers of butyrate dopes offer a matching range of synthetic enamels or acrylics for this purpose. Two important points to remember if using butyrate's are:
- Butyrate dopes adhere well to nitrate dopes, but the reverse is not true.
- If an old butyrate finish requires a paint repair, it is often necessary to pre-treat the area with a slow drying butyrate solvent called ‘Butysolve’ prior to applying new butyrate in order to achieve good paint adhesion.
Finally, there are modern ‘two-pack’ polyurethane finishes. Loved by some builders and loathed by others, they offer a real alternative to traditional air drying finishes and are usually substituted for the final finishing coats over traditional base coats. The main disadvantages are:
- The ultra-high gloss finish. If the surface is not as good as you would have wished a high gloss finish will accentuate every imperfection.
- The flexibility. Because polyurethanes can be made to remain permanently flexible they are difficult to rub down if runs and drips appear in the finish.
- The weight. They are by comparison with cellulose dopes, HEAVY. Because 2 pack paints cure mainly by chemical reaction rather than solvent evaporation, the weight of the paint tends not to decrease after application. This is unlike nitrate dopes, where in some cases, about 80% of the paint weight is made up of evaporating solvents.
- They require a different spraying technique. Because the cure time tends to be longer than air drying paints, there is more time for runs and sags to appear.
- Polyurethane's often use poisonous agents in either the paint or the hardener, so it is vitally important to use correct respiration equipment.
Well, those are the problems. These characteristics (apart from the last) also form the basics of the main advantages of this type of paint:
- The ultra high gloss finish. Providing you have done a good job with the etch primer coats, polyurethane's will give a very high gloss finish without the need for cutting and compounding that will remain glossy with minimal care for many years.
- The flexibility. Flexible polyurethane's remain flexible indefinitely and tend not to crack like traditional dopes.
- Although heavy, the pigment density tends to be far higher than with dopes, giving very high covering power with a thin coat.
- The different spraying technique required for polyurethane's is easy to learn and merely requires several light coats with an appropriate ‘flash off’ time between coats and a more accurate control of thinners to paint ratio.
- All paints are poisonous to some extent, so why not protect your health and use a proper air-fed mask regardless of which paint system you finally decide upon.
Just one thought about the polyurethane versus dope question. If you manage to damage the fabric on your polyurethane finished aeroplane, you will have to remove the paint in the local area of the damage in order to stick on a repair patch. But you don’t intend to damage your new, beautifully painted masterpiece, do you?!
Our sincere thanks to Rob Millinship / Aero Graphic for allowing us to reproduce this article of his which we hope will help the uninitiated in that difficult task of completing the final stages of building an aircraft.
Rob is always willing to give some advice to those in need of assistance and can be contacted on 01159 290315 or http://www.aerographic.com