Let us spray

Let us spray

I’d been itching to get some paint on my Cessna Bobcat, which was adorned with rib stitching and pinking tapes. But what paint to use? Regular readers may recall my review (February 2006 RCM&E) of Klass Kote, a very durable, fuelproof 2-pack epoxy system from the USA that’s imported to the UK by Baron Paints. I was so impressed with this paint that I decided to use it on the Bobcat, and a ‘phone call to Wayne Fryer at Baron Paints soon had paint, catalyst and thinners on its way.

TOOLING UP
I started spraying models many years ago with very cheap, basic equipment and soon discovered that using such budget hardware gave poor results. Since then I’ve invested in a good quality, regulated compressor that can provide the constant pressure required to operate decent quality spray guns and so produce a good finish. The compressor has a 11/2hp motor that compresses the air in the reservoir to 120psi, which can then be regulated down to the pressure needed to operate the spray guns (typically 20 to 50psi for the guns I use).

I’ve a collection of several spray guns of different types, and I find that gravity fed guns (with the paint reservoir mounted on top) are more than adequate for the size of models I build. These are often referred to in the car repair business as ‘touch-up’ guns and are smaller than the ‘siphon’ variety that have a very large can on their underside.

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Needless to say, spray painting needs to be approached with health and safety in mind as a lot of paint finishes up in the atmosphere and can be inhaled unless you take the necessary precautions. A normal dust mask will remove some of the paint particles but it won’t stop the fumes getting into your lungs, so it’s essential to use a good quality mask that incorporates a fume filter. Also, try to paint where there’s some circulation of air to remove the build up of overspray dust and fumes. Spraying outdoors may seem the obvious answer here, but even on a calm day this proves near impossible due to the excessive movement of air. I use my garage, with the ‘up and over’ door and side door slightly open, adjusting as necessary to suit the strength of any prevailing breeze.

My DeVilbiss airbrush – used for spraying the reg' numbers onto the fuselage sides.

It’s said that the old type cellulose paints, if breathed into the body, will break down and pass through your system within a couple of weeks. However any polyurethane or epoxy-type paint that’s inhaled will remain in your body, so it’s important to make sure it doesn’t get there. My advice is to take precautions whatever medium you’re spraying.

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START RIGHT
To get a good finish you need to start with the right colours to avoid an unnecessary build-up of paint (and therefore weight); always try and start with the light colours first, as dark colours will cover these far easier.

I based my Bobcat’s scheme on a restored example that I saw back in the late 1980’s whilst on holiday in Florida: top half of the fuselage white, lower half plus wings, tailplane etc. in turquoise, engine nacelles and trim around the cabin windows etc. in black. An attractive colour scheme that should look very distinctive in flight.

Starting with the light colours, then, the obvious place to begin was the white top half of the fuselage… at least that’s what I thought until I looked closer at the black trim line around the fillets on the tailplane and base of the fin. To cut and apply parallel masking around those edges was going to be very difficult, so I decided to hand-paint the lines a little oversize and then mask them to give the required result once spraying was completed. I’d discovered from previous experiments that white Klass Kote would cover black very well indeed, so I had no qualms about hiding the small amount of black at the trim tapes. I usually find that Sellotape is ideal for masking, but this wouldn’t follow the curves of these trim lines so I opted to use electrical PVC insulation tape instead. Lay this onto a clean, smooth, flat surface, take a straight edge and a sharp scalpel and you can cut trim tapes to any width you want.

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The Clarke gravity fed spray gun that I use for larger areas.

Before you start spray painting it’s a good idea to clean down your workshop area as much as possible because once you start, the draught from the spray gun can disturb any dust that’s lying around and you can guarantee it will finish up on your wet paint surface. An advantage with spray paints such as epoxy and cellulose is that they’re quick drying, giving the little dust particles just five or ten minutes to get themselves permanently stuck to the paint. Any dust that lands after the paint has ‘tacked off’ should be quite easy to remove with a light rub of a rag once the surface is thoroughly dry.

Looking way back I remember spray-painting a model with Humbrol enamel whilst wearing a woollen jumper. This was a slow drying paint, and the model looked perfect when I’d finished. But when I examined my work the next day the fluff from my jumper had attached itself to the wet paint, giving the model a ‘mohair’ effect! Since then I’ve made sure to wear nothing fluffy whilst spraying; an old cotton shirt or cotton jacket is just the job.

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APPLICATION
Klass Kote is initially mixed one part paint to one part catalyst. There are gloss and matt catalysts available, so if you wish to produce a mid-sheen you could use two part paint, one part gloss catalyst and one part matt catalyst (or any ratio that maintains the 50:50 mix for paint and catalyst). If spraying, then this mix needs further thinning to suit your spray gun pressure etc.; anything up to 50% would be the normal starting point. Instructions supplied with Klass Kote suggest mixing the paint and allowing it to stand for 30 minutes before spraying, giving it the occasional stir during that period.

Before spraying there’s one more thing to consider. When the parts have been painted, how are you going to hang them so that they don’t touch anything whilst the paint dries? I usually have a selection of  ‘S-shaped hooks made from piano wire, and nails stuck into the edge of shelf or ceiling rafters for hanging things like ailerons and elevators.
Okay, time to get spraying. Two coats of white paint were applied to the top half of the fuselage and when dry this was masked for the lower half to be sprayed. The main masking was done using 1/2” wide Sellotape, which is very good for straight lines. I’ve experimented with many types of tape over the years but always come back to good old clear Sellotape. If you lay a length on a painted surface and lightly press it down with your finger it will stick fairly well; if you then rub a fingernail over the edge of the tape it sticks down with more authority, where paint might otherwise creep under. With the tape in place one then needs to cover the rest of the model with newspaper, which can be held in place using regular masking tape, stuck on top of the Sellotape.

Electrical PVC tape can be cut into narrow pieces to make masking strips that go around curves very nicely.

Klass Kote is available in quite a good range of colours and there’s actually a turquoise blue, but in comparison to the photos of the full-size Bobcat it has too much green content, so I decided to mix the paint from scratch.

The Bobcat’s wings are detachable from the fuselage and so didn’t require any masking when applying the turquoise. When this had dried the wings were masked so that the engine nacelles could be painted black. The fuselage required quite a bit of additional masking as well; all the black trim lines needed carefully picking out, most of which was done with Sellotape, although some of the finer lines were done with PVC electrical tape as described earlier. On closer study of the photos I realised that there was a very thin line, which on the model would be only 0.5mm wide and near impossible to mask as any variation in its width would be extremely apparent. To achieve a line of consistent width I used a pen I picked up many years ago, which has a small reservoir. With some thinned black paint loaded and a little experimentation, running the pen along a straight edge produced a clean, thin straight line. I also found it best to hold the straight edge in place with masking tape so that I could concentrate on scribing one continuous line with the pen.

NUMBERS GAME
All aircraft have registration numbers on them somewhere, varying in size from large (as seen on the wings of a Tiger Moth) to small (as on the Bobcat; which has its registration under the tailplane). The American military tend to use letters and numbers that are drawn on a grid, and it appears this system was used on the Bobcat. When building my Beech 18 twin I drew the complete alphabet plus numbers 1 to 9 on the computer using the American-style grid, so it was fairly easy this time around to open up the CAD package and arrange the registration ‘N-69072’ on the computer and scale it to the required size before printing it out on paper.

The lettering print out and roll of Frisk film prior to my little light box trick.

I needed a spray mask for the registration, which I made using Frisk film, a low-tack, self-adhesive masking medium that’s available from graphic media suppliers in both sheet and roll form. Laying the computer-printed registration onto my home-made light-box and taped Frisk film over it. With this, the printed registration became very clear through the masking film and with a new blade in the scalpel, plus a steady hand and straight edge, I cut out the letters to produce the two sets of masks that were required. These were then carefully stuck in place under the tailplane with newspaper masking around them to stop overspray getting on the rest of the model. The registration was then painted using black gloss paint through a fine airbrush.

SPRAY TIPS
When I first started spray painting I didn’t get very good results. However, I learned a few lessons fairly quickly and hopefully the following advice will help you avoid making the same mistakes I did!

When spraying, always keep the spray gun a constant distance away from the object being painted. Reducing the distance results in a greater concentration of paint, whilst increasing the distance reduces the concentration; this, of course, produces uneven results.

Start spraying before making contact with the surface being painted, and stop only when you’ve cleared it. For example, when painting a wing it’s best to start spraying a fraction before you get to the wing tip, approaching the tip with the spray gun in full flow, carrying on down the length of the wing and release the button only when you’ve cleared the other end.

When you’ve applied your first coat of paint and allowed it to dry, check for any imperfections. Any high spots can be rubbed down using fine grade wet-and-dry paper used wet. 600 or 1000 grades are ideal for this, as they’ll produce a very fine finish. Any low points can be filled using cellulose putty, which is basically a thick cellulose grey primer that can be worked with a putty knife. Depending on the thickness applied this will take a few hours to set before it can be rubbed down and blended in.

With a little practice you should quickly get good results, but only if you buy a decent quality spray gun and the proper compressor. When you’ve finished spraying it’s very important to clean the gun properly. Strip it down, clean all the parts with the appropriate thinners and then reassemble it. Pour some thinners in and spray through the gun at least twice. Be thorough here, as any paint left in the unit will set and be the cause of many problems next time you try to spray.

 

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