Intro:

Zunum Aero is a cutting edge aeronautical engineering firm in Kirkland, Washington that’s developing hybrid electric propulsion systems for large aircraft. Their first plane is to be a 12-seat regional airliner with a range of 700 miles.

Yup, that’s right an electric airliner. This technology could fundamentally change the aircraft industry by simultaneously reducing costs by half and emissions by 80%. Recent articles on this pretty amazing company and its technology can be found here and here.

On August 30th of this year Matt Knapp, CEO and Founder of Zunum Aero, contacted the President and V.P. of Seattle IPMS hoping to find someone to paint and finish a 3-D printed display model of their brainchild. The challenge was the model had to be completed for a media blitz scheduled to start September 12: just 13 days later.

The request was relayed to some of the more experienced (read old) members of the Seattle IPMS membership. I responded and after a phone chat, Matt and I were scheduled to meet on Saturday the 2nd of September to review the model at Fathom, a 3-D printing service in Seattle.

It was immediately obvious upon inspection that the crew at Fathom did an awesome job on the printing, assembly, and wet sanding of the model. Even in model form the prototype is imbued with particularly pleasing lines including an elegant V-tail. Luckily for me, the surface of most of the model was very smooth and looked as though a minimal amount of preparation would be required for painting.

Unfortunately, there was an overriding issue: about 1/3 of each ruddervator was missing as a result of incomplete printing. This was a simple oversight that would have been easily remedied by another round of printing. However, Monday would be Labor Day and the Fathom crew had gone home for the long weekend and wasn’t scheduled to return until Tuesday.

Uh-oh…………………………..10 days left

The Zunum Challenge

With ten days before the start of the media blitz, I placed the model on my bench and began developing a strategy for fixing the tail and getting the hummer into paint in what was a desperately small amount of time.

What follows is an account of those efforts illustrated with pictures that are admittedly not of a very high standard. In the effort to “get ‘er done,” corners were cut including photography. I therefore I ask your indulgence.

Restoring the V-tail:

As chance would have it, I have a thing for V-tail designs so I was motivated to make this one right. Each ruddervator was lengthened using extensions made from laminated sheets of styrene of differing thickness. This was done in order to achieve the approximate thickness and contours of the stub onto which the extensions would be glued.

If successful, the extensions would require a minimum amount of blending with putty and sanding once in place: a good thing. Most importantly, the length and contours of the extensions were formed following additional drawings and measurements provided by Matt Knapp.

After the extensions were glued in place with CA, they were blended in by sanding with Alpha Abrasives 400 and 600 grit paper followed by a coat Mr Surfacer 500 along the join. When dry, the extensions were again blended with Alpha 600 and 1,000 grit paper followed by 3200 and 3600 micromesh cloth used with cold soapy water. A coat of Mr. Surfacer 1,000 was then applied and when dry, this was buffed out with a sheet of Micromesh 3600.

With the tail extensions in place I set about getting the model into primer. Most of the models surface was sufficiently smooth however there were areas that needed filling and blending. Moreover, some of the more curved surfaces were slightly faceted and had to be sanded down and smoothed.

The faceted areas were smoothed with Alpha Abrasives 400 and 600 grits for the initial sanding steps then with Alpha 1,000 and 4,000. Any area that was sanded was subsequently smoothed with 3200 then 3600 Micromesh sheets and/or pads depending on the area.

In addition, the outer ~3/4 of each wing was glued to a wing stub on the side of the fuselage. Both wing joins required filling and blending with Micromesh 3600. Most importantly, the wings (attached by a smooth butt-join with no tab or support) and were affixed with two-part epoxy.

The wingspan on this model was 2 feet and the wings were solid meaning there was a fair amount of weight and stress on these epoxied seems: more (much, much more) on these later.

Getting ‘er into primer:

Prior to shooting a primer coat, the entire model was wiped down with a Kimwipe saturated with denatured alcohol. Since many areas on the model required filling, I opted to use grey Dupli-Color Automotive sandable Primer. This is a lacquer-based primer with a pretty heavy filler that dries fast and sands quite nicely.

The primer was decanted from a spray can through a straw into a 50ml beaker. Prior to spraying, lacquer thinner was added to about 10% by volume. This was sprayed at 18psi with an H&S Evolution fitted with a 0.4mm tip and a 5ml paint cup. Occasional stirring of the mix in the airbrush paint cup was required as there’s a lot of filler in this primer. Moreover, this was a large model and the option of using a 0.4mm tip allowed me to lay down a heavy yet smooth coat of primer.

Being a lacquer, Dupli-Color primer dries quickly which allowed me to start wet sanding the entire model with Alpha 1,000 grit paper followed by Micromesh 3600 within 2 hours of shooting. The model was given three coats of primer with a wet sanding after each.

A final inspection revealed that some areas still required primer to fill the facets. In addition, the intakes on the engine nacelles and engine mounts required additional filling with Mr. Surfacer 1,000. All of these areas were given a final coat of primer and a wet sanding before the finish was deemed sufficiently smooth for paint.

After the Dupli-Color primer was allowed a full 12 hours to dry, the entire model was sprayed with Mission Models White Primer diluted 30% with Mission Thinner. This was shot at 15psi with an H&S Evolution fitted with a 0.4mm tip and a 5ml paint cup (see Technical Note 1: Diluting Spraying Mission Models Primers, below).

The Mission Primer was allowed to dry for about 2 hours then the entire model was wet buffed with Micromesh 4,000 sheets and pads. This was done with cold soapy water and frequent wipes with a clean Kimwipe to keep from scratching the finish while buffing.

After a thorough inspection, some areas were shot with another coat of Mission White Primer (described above). These areas were allowed to dry for ~2 hrs aided by a strategically placed hair dryer then buffed again with Micromesh 4,000 and soapy cold water.

And then the wing cracked………………………

While buffing the Mission Primer a crack developed on the lower surface of the port wing along the epoxied seam. Needless to say, this did not make me happy camper.

I sanded through the layers of primer with Alpha 320 and 400 grit paper to the models surface and removed epoxy from the crack with an X-acto blade and dental pick. The crack was filled with CA that was then set with accelerator (Bob Smith).

When dry, this was dressed and sanded smooth with Alpha 400, 600 and 1,000 grits followed by a coat of Mr. Surfacer 1,000 over the repair. When dry, the Mr. Surfacer was smoothed with Micromesh 3200 then 3600 and the area was re-sprayed with Mission White Primer applied with an H&S Infinity fitted with a 0.2mm tip. When dry, this was blended with the surrounding areas by more wet buffing with Micromesh 4-6,000.

With the wing repaired, I set about cutting stencils and taping the model off for it’s first coat of color. The date was Thursday, the 7th of September: 5 days left. I asked my wife to buy more coffee, a lot more coffee.

Stencils and Tape:

As a guide to painting the scheme, I was provided PDF files showing multiple views of the aircraft that, when enlarged appropriately, were scaled to the model. These were used to make stencils for the side windows, portions of the cockpit windscreen, and the wavy blue ribbon on the fuselage sides. The stencils were cut over a glass-covered light box from sheets of Kabuki tape material (Tamiya).

In anticipation of issues with the N number, I had previously reached out to Decal Czar Sir Kenneth Murphy, a fellow member of IPMS Seattle. Sir Ken was able to produce decals of the Zunum hummingbird logo, doors, and N number from scans I emailed him and did it in short order. Many, many thanks, Sir Ken.

Shooting Paint and then………………….

The windows and cockpit windscreens were taped off and sprayed with a mixture of Mission Models Black and Blue at an approximate ratio of 8:2. The paint was diluted with Mission Models Thinner/Polymix at a ratio of 60:40 (Paint to Thinner/Poly) (see Technical Note-1 below: Diluting and Spraying Mission Model Paints). This was sprayed at 12psi with an H&S Infinity fitted with a 0.2 mm tip.

When the paint was dry (~10 minutes), I began removing the tape and stencils and heard a sickening cracking sound as the other epoxied wing came cleanly off the model along with the piece of tape I was tugging on. It was at that moment I invented an entirely new kind of profanity.

Repairing the wing…again:

The surfaces of both the wing and stub were sanded with Alpha 400 and 600 down to the models surface and, once again, I removed dried epoxy with an X-acto blade.

To give the glue more surface area for the repair, the mating surfaces were roughened with a dental burr and the wing re-attached with a mixture of CA glue and talcum powder that was fixed in place with accelerator (Bob Smith).

The join was then sanded smooth with Alpha 600 followed by 1,000 grit paper and “painted” with a thick coat of Mr. Surfacer 1,000. When dry, the Mr. Surfacer was smoothed with Micromesh 3200 then 3600.

The repaired area was re-sprayed with Mission White Primer applied with an H&S Infinity fitted with a 0.2mm tip. Multiple dry coats were applied followed by a heavy wet coat for optimal blending with the surrounding areas and coverage. When dry, this was blended further by wet buffing with Micromesh 4,000 and 6,000.

Back to Paint:

With the wing repaired, the model was taped off for the green stripes on each side of the fuselage. The green paint was custom mixed using Mission Models Yellow, Blue and White and applied at 12psi with an H&S Evolution fitted with a 0.2mm tip. This was shot in the early morning hours of Saturday, September, 9th: 3 days left.

Later that same Saturday morning I received decals of the Zunum logo, hummingbird, doors, N number, and windows from Sir Kenneth at our monthly IPMS meeting. Racing home, I set about matching the blue of the hummingbird decal using a custom mix of Mission Models Blue, White, and Grey.

With the blue matched, I applied the previously cut ribbon stencils on each side of the fuselage. The blue (and green) patterns on rear of the fuselage and V-tail were masked free hand (no stencils). This was a fair amount of masking that took way longer than anticipated. The blue was shot at 12psi with an H&S Infinity fitted with a 0.2mm tip (see Note-2 below: Diluting and Spraying Mission Model Paints).

In preparation for being painted silver the areas around the leading edges of the wings as well as the lips of the dorsal intake and engine nacelles were masked off with Tamiya tape. These areas were then shot with a 1:1 mixture of Mission Models Aluminum and Steel to which a few drops of Mission Models White was added. This was sprayed at 12psi with an H&S Infinity fitted with a 0.2mm tip (see Note-2 below: Diluting and Spraying Mission Model Paints). With the painting done, I moved on to shooting a gloss coat.

Mission Models Gloss Coat:

In preparation for decals, the entire model was shot with Mission Models Gloss Clear Coat. This is an awesome gloss coat that gives a great finish when dry and is exceedingly hardy. So hardy, it can be used on RC models because it’s resistant to fuel. That being the case, it’s impervious to both Micro-Set and –Sol: my go-to decal solvents.

Diluting and spraying Mission Models Gloss Coat

For spraying, I dilute Mission Models Gloss with Mission Thinner at a ratio of 30:70 (Gloss:Thinner) and shoot it at ~15psi. Given the size of the model, I opted to spray it with my H&S Evolution fitted with a 0.4mm tip and a 5ml paint cup.

With the gloss coat shot, I set the model aside and fixed another cup of coffee. It was Monday, September 11th: ~24 hours left.

Turning Final:

Sine the decals had been printed on clear decal paper they had to be sealed with a clear coat prior to emulsion in water or they would disintegrate. I’ve not used this type of decal often so I decided to test my techniques (and clear coat) on a spare decal.

In addition to all the aforementioned decals, Sir Kenneth (The Decal Czar) had kindly printed decals of all the side windows. Unfortunately, I had already painted the windows before getting the decals. However the extra window decals gave me material to experiment with.

Through trial and error I found that Mission Gloss delivered as a wet coat caused the rapid disintegration of the decals. Conversely, if the Gloss was first applied as a dry coat and built up gradually, the decals weren’t affected.

With the technique established, all the decals were shot with Mission Gloss, cut from the decal sheet, and applied using Micro-Set, -Sol, and conventional techniques. They worked beautifully: not one problem and yes, I really needed the break.

After giving the decals as long as possible to dry (hastened by the careful application of a hair dryer) the decals were given a coat of Mission Gloss to protect them while handling the model. I then set about adding lines for all the control surfaces (aileron, flaps, ruddervators) using a 1mm wide black graphic arts tape which I’ve had in my stash for many years waiting for the opportunity to use–finally.

With the control surfaces complete, the entire model was given a final heavy coat of Mission Models Gloss diluted and sprayed as described above. The model was then set aside to dry and I set all my airbrushes up for a long soak and thorough cleaning.

The date was Tuesday, September 12th.

It was ~3:30am: 5 hours to spare.

I made some coffee and watched a Perry Mason on my DVR.

Conclusion:

With the model safely packed in the back of my car I pulled up to the corporate offices of Zunum Aero at around 8:00am. The model was unboxed, mounted on it’s stand, and presented to the Founder of Zunum Aero, Matt Knapp, who was visibly pleased with the result: mission accomplished. The first of many media events was scheduled to start later that day and went quite well as I was later informed by Matt. Whew………..

In 40 years of modeling I’ve never painted such a complex scheme on such a large model in such a short period of time and I hope to never do it again:) That said, the opportunity to meet and briefly work with Mr. Knapp and assist, in an ever small way, the mission of Zunum Aero, made it worth the while.

Good luck Zunum Aero!

Now go paint something!

Special thanks to Matt Knapp of Zunum Aero for the green-screen images of the completed model.

Technical Notes:

1) Diluting and Spraying Mission Models Primer:

A) Quick and Easy

As reccomended in the Mission Models instructions for primers, an easy starting mix for general spraying can be obtained by adding the following to your paint cup:

 10 drops of Mission Primer

 3-4 drops of Mission Thinner

 No Polymix Additive should be added to Mission Primers!!!!

Mix in your paint cup, dial in ~15 psi on your compressor, and prime away 🙂

B) Measured by Volume:

After playing around with multiple dilutions, I found that I prefer a ratio of Primer to Thinner of 60:40. That’s 60% Primer, 40% Thinner.

2) Diluting and Spraying Mission Models Paint:

A) Quick and Easy

As recommended in the Mission Models paint instructions, an easy starting mix for general spraying can be obtained by adding the following to your paint cup:

 10 drops of Mission Paint

 2-3 drops of Mission Thinner

 2-3 drop of Mission Polymix Additive (a leveling agent).

Mix in your paint cup, dial in 12-15 psi on your compressor, and spray away 🙂

B) Preparing a Pre-Mix of Mission Thinner and Polymix:

To dilute and spray Mission paints I first prepare a solution of Mission Model Thinner to which Missions Polyurethane Mix Additive has been added to ~10% by volume (or a 10% solution). Mission Polymix is a flow and leveling agent that greatly decreases tip dry and dry over-spray: good stuff. I then dilute Mission Models Paints directly into this 10% solution for spraying.

Using a pre-mixed solution of Polymix and Thinner allows you to prepare paint dilutions in a much more reproducible way ensuring that the mixes will spray in a predictable manner. In short, why re-invent the “the paint wheel” every time you mix up a batch of paint because you missed counting some drops and/or the drops are of different sizes? Diluting all your paints with Thinner-10 simply decreases batch-to-batch variability.

If counting drops is how you roll however, no worries. Thinner-10 is roughly 1 drop of Polymix to ~10 drops of Thinner. If you prefer to work in ratios, Thinner-10 equates to a ratio of Polymix to Thinner of 1:10. I prepare a large volume (40ml) at a time and store this in an appropriate bottle for use when painting. Properly stored, Thinner-10 lasts for many months (forever?) with no issues.

Preparing Mission Models “Thinner-10”

1) Using a 3.5ml pipette, transfer a total of 4ml of Mission Models Polyurethane Mix Additive to a 50ml beaker.

2) Add Mission Models Thinner up to the 40ml mark on the beaker.

3) After stirring the mix with a spatula, transfer the entirety into a 40ml paint storage bottle.

General Spraying:

Dilute Mission Model paints directly into Thinner-10 at a ratio of 60% paint to 40% Thinner-10. Adjust your pressure to 12-15psi and spray away 🙂

Fine-Line Work:

Dilute Mission Model paints directly into Thinner-10 at a ratio of Paint to Thinner-10 of 40:60 to 30:70. I give a range here, as there will be variability in the viscosities of the paints as well as how thin a line is desired. Thinner lines require more dilute mixes. Experiment with different dilutions to identify one that suites your application. Adjust your pressure to ~10psi and spray away 🙂

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