The inspiration, the philosophy, and the story behind the Jart can be found on the Jart World Web page. Well worth a visit!

Wednesday, December 19, 2007

Journey to JARTdom - Part 2

Making a plug
During one of the many mold making courses that I attended, I learnt that the first step towards JARTdom was to constructing a fuselage mold – a relatively easy, but somewhat painstaking task. I learnt that one approach was to use a medium grade wood for the plug (& I just happened to have some blocks of local Maranti handy - a medium hardness wood with a relatively close grain) from which a “female” plug is made. Later I decided that this wood grain was too course, & would rather recommend something with finer grain & somewhat softer density. After some research I found the correct version of the freeware Adobe Reader 6.x and printed the plans out using the Tiling feature (this feature is only available in the commercial version of later releases) onto A4 sheets & stuck these together. Soon this was transferred to the wood blocks that had been joined with a PVA wood glue (Lesson: this was a bad choice – as the water used during fine sanding wicks eats into this join & it creates a small mark along the plug – if you are thinking of doing this, use a waterproof glue).
I used a powered jigsaw and a plane to rough shape the blocks, after cutting templates from cardboard. The side profile looked awesome, but the plan view just did not “feel” right, so I modified it a little such that the nose was slightly narrower than the original 8020 shape. Then I applied lots, LOTS, actually HUGE AMOUNTS, of sandpaper until I had something that resembled a relatively smooth block of wood, which had even started to look like a JART fuselage amongst the copious quantities of wood shaving & dust. Did I mention that I used a small warehouse of sandpaper? I now understand why molded aircraft cost so much – it is all the sandpaper that is used.

First steps, the wood blocks are joined, rough shaped,…
... and liberal sanding is applied to create a smooth surface
Then came the interesting task of creating a smooth surface that would yield a polished “glass finish” type model – the stuff you see on all those fancy imported models. I started with common garden variety sanding sealer – boy this stuff smells just like the dope we used with tissue paper on those small free flight models we built as kids. It gives you the same headache too if you paint in a closed area.
After the first coat, the wood was all hairy again, and it was back to sanding until a silky “soft” finish is achieved (round about 400 grade paper). I then switched to water paper (600 grade) and had a “soapy” smooth finish after some effort. At this point I started to think that the building group would have to sponsor pay my medical bills for my tennis elbow! Around this time I also learnt the error of my ways when I elected to use a non-waterproof glue to join the smaller wood blocks, since the water used during sanding seeped into the join & created a small ridge, no matter what. Only dry sanding & later sealing eventually cured this.
After repeating the sealer & sanding most of it off again process for the third time, I sprayed (from a spray can) the first application grey primer paint & again removed 99% of this in the sanding process too. After I had used the entire can of primer (which equated to approximately 3 full coats sprayed on & subsequently sanded off). Just before the final sanding sealer coat & primer was painted, I “added” the 12mm thick balsa fin & had this smooth in no time at all. Unfortunately, I must have gained considerable upper body strength in the previous fuselage sanding process & proceeded to over sand the fin – which was now also a little sleeker than the original section, so I thought “why not?” and stuck it in place. I then applied a fillet of SikaDur epoxy between the fuselage & fin, and with a wet finger smoothed this until it also “looked right”. Sanding time….
I was struggling to remove the grain effect – the wood I chose was a little too course grained & water paper sanding down to the wood layer resulted in fine holes appearing. At the suggestion of an experienced friend, I mixed 10%-20% talcum powder into some sanding sealer & applied this as a surface sealer, before sanding it all smooth with 600 grade water paper. I also learnt the trick of adding a little dishwashing liquid to the water as this prevents instant clogging of the sandpaper. Finally I learnt that you have to wait until the sanding/sealer/talc combination is 100% dry – as tiny little bits are created otherwise which create long grooves in a nearly smooth surface. Oh well, nothing that more sealer & sanding could not cure.
My friend Evan (of F3B - Shongololo building group fame) advised me to use an automotive 2 part paint to create the best surface finish on the plug. So, Marcel, who had planted the orginal seedf, and who had sprayed 2K onto his aerobatic F3A models before, was lured over to the house with the promise of beer. He was soon set the task of spraying the plug (I looked after the beer part of the deal). The only mistake I made was to choose white paint only. If I ever do this again, I would spray layers with alternate colours (like white & black), as it really speed up identifying small holes/bumps during subsequent sanding processes. When it was my turn I learnt all about overspray, pinholes, orange peal & various other interesting “effects” that are achieved when spraying. Fortunately, and once again, it was nothing that a little sanding could not cure. In fact I have come to learn that sandpaper is your friend – my local sandpaper wholesaler provided me with a great wall calendar & even the recommended retail price list for 2008!
The 2 part automotive paint used to achieve the final surface finish
Around 3 days after spraying the frist coat of 2K, I started using 800 grade water paper, and admit that the water & dishwasher liquid approach resulted in a serious saving of the rain forest as I was now only using a few parts of sheet per layer (dunno what the carbon footprint of dishwashing liquid is, but being glider guider, I reckoned I already had to have some positive eco-points stored up already).

Post 800 grade waterpaper sanding – a smooth sheen is starting to emerge

After two more coats of the 2K paint I had almost eliminated all the grain marks & join lines completely. I finally switched to a 1000 grade water paper & managed to get a “buffed” surface finish – if you wipe a wet cloth over it, it shines. I thought that the time for polishing had arrived. Using a sponge polishing pad borrowed from Evan, and some G3 polishing compound, that matt surface was quickly changed into a shiny “new car” finish & at last I had a plug that I was proud of.

I took it along to the BERG club (current home of the F3B Shongololo building group) for a show & tell were we fooled around a little for the photo’s. The list of aspirant building group members has started to grow (note to self: I will have to create a formal list once the molds are ready).

Next stage: building the mold

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