Wax on, Wax off

I wish the title were referring to actual wax.  Nope, still doing body finishing work.  Spray paint on, sand off most.  Fiberglass and epoxy on, sand off most.  Now glazing putty on, sand off. most  I have more than a few more rounds of sanding to go and spray paint to go.  It's going to pay off though when the parts releases easily from the mold and doesn't need much of any finishing work.

Wax on (glazing)

Wax off (sanding)

Much more sanding to do.

Early seat mold finishing work

More body finishing work today now that the spray paint has dried.  It took a couple hours of wet sanding with a small flexible block to knock off most of the heavy scratches.  This is by no means flat, as you can see by the paint color remaining in streaks.  To really get it smooth, there is a lot more finishing work needed.  How smooth does it really need to be for a seat?  It won't be a production item, so I'm okay cutting corners in overall smoothness quality.

Instead of another round of paint, which wouldn't really fill in the deeper areas, to add a layer of tinted epoxy and fiberglass.  This serves two purposes: 1. Provides some thickness for smoothing the surface, 2. Seals any holes & cracks around the joint between the Bondo and plastic (air leaks would be bad during bagging).  So far, I only added a single layer of 1.4oz glass, and that's not much thickness.  Before going to be tonight, I'll add another layer of raw epoxy to fill the weave and provide a bit more thickness for sanding off after it cures.  This has the beginnings of too much work...

I also laid up a 2x2 inch test coupon of the candidate 1.4oz + (3x) 6oz + 2mm Soric + (2x) 6oz glass layup schedule.  The Soric certainly took as much resin as I threw at it, but otherwise was quite nice to work with.  It'll be interesting to pull the coupon out from the vacuum bag tomorrow and feel how flexible it is.  Cutting to a specific, known size, I can also get a rough idea of the total seat weight by making an educated guess at the seat's area.  The original go-kart seat weight was 15lb and the half-seat weight was about 6lb.  If I can come back to 6lb with a whole lot more stiffness, I'll consider that a win.  Certainly I can keep pulling weight from the seat by varying the lamination schedule and core material.

Updates on the seat

After being gone all week on travel for work (last week), it's great to be back working on a beautiful spring weekend (last weekend).  After a lot of fitting of the plastic half-seat on the fuselage, I have decided the right answer is to make some additional modifications in preparation for using it as a mold for a composite seat.  The plastic half-seat could probably work fine on its own with some mounting bolts to be honest, but I really want something that I feel confident in.

I wanted to remove the headrest on the original seat without killing my option to include it later.  So I added a cardboard and Bondo dam that will serve as the new top of the seat, but leaves the option to remove the dam and remold the seat if I'd like a headrest later.  The old hole through the seat back also has to be erased since that area needs to sit flat against the upper seat tube.  I'll later cut a new, smaller slot for the seat belt, which will be located after the seat is molded and bolted to the nose frame.

I'm adding stand-offs to the mold so the seat will have built-in attachments for the mount tubes.  These will be recesses in the seating surface and can be hidden with a seat cushion.  After getting the tube alignment by blobbing on Bondo, sitting the seat in place on the nose, and popping it off, further build-up can be done using a spare tube.  I like to trim Bondo when it's still green, after it stops being sticky, but before it's too stiff for a knife to cut through.  The consistency is like clay for about two minutes, and that's when it's easiest to do shaping.  After that stage, it's better to let it cure for 10 minutes and come back with a Dremel or sanding block.  I did wrap sandpaper around this tube and open the diameter of the mounting flange, leaving a gap of approximately the expected laminate thickness in this mounting area.

I still have to do some clean-up in the two side seat belt holes and then go around trimming plastic along the whole rim to ensure there is draft everywhere around the edges so the part will release from the mold.  I'll probably also build a 2x4 frame / table for the seat to make the mold easier to work with.

And I did build a frame after all.  Again Bondo (polyester resin) is used to mount the seat mold to the frame.  Why?  Because it's easy and super fast.

So next comes the boring part of preparing the seat mold surface finish.  The real point is to make the surface release from the part.  There are a few easy ways to do this and many hard ways.  An easy way is to cover the whole surface in thin packing tape (there is a good duct foil tape that I like).  That tape surface can then be waxed, PVA'd, and is ready for molding.  The bummer is that every last tape edge will show in the final part and must be sanded out: more body work on the final part.

A harder way (and of course the path I'm heading down now) is to seal the whole surface and polish it smooth.  The raw polyethylene Rotomolded surface was smooth and would probably have released with just a waxing.  But there were many areas of plastic that I ended up sanding as part of the body work process.  Those areas have to be smoothed out and polished.  Thinking I couldn't sand the plastic back to acceptable polish no matter how hard I tried (it seems to get fuzzy), my thought is to seal the surface with a spray paint and simply polish the paint layer.

I had some Valspar spray paint that is marketed for plastic.  After cleaning with mineral spirits, a test spray showed it covered nicely.  On the raw plastic areas, it scraped off with just a fingernail scratch.  There would be no way to sand that to a smooth polish without it flaking off.  On the other hand, the areas that had been sanded already stuck very well, given that they already had some tooth for the paint to grab. 

So I sanded the whole surface with 80 grit sandpaper, including going over the areas I had already test-painted.  After spraying almost an entire rattle can of paint, the surface looks uniform and definitely shows every scratch and divot.  It needs to dry / harden overnight before I dare start sanding.  This is the boring part though, sanding, filling, spraying, sanding, filling, spraying, sanding, sanding, sanding...  I take normal molds to 2000 grit, but this should probably release okay at 600 grit.  Assuming all the divots, scratches, and other surface features can be worked out, I'll be able to lay down a surface coat of gel coat and not have much body work to do on the final part once it's trimmed.

The next fun debate is the layup schedule of the final seat part.  I am open to suggestions here.

I have some 2mm Lantor Soric XF material I'm thinking of using as a core.  It's designed for resin infusion, which I haven't done before and don't plan to try for this (though it would be appropriate), but I think would work okay for a wet layup.  I have used Lantor Coremat XM before on another project of this size, and it conforms very nicely to 3d shapes.  However, Coremat absorbs an awful lot of resin.  The Soric hexagonal cells seem stiffer than the Coremat, so I'm hopeful they absorb less resin overall, yet it seems quite reasonable to work around the 3d curvature this seat has going on.  By the datasheets, Soric is better, but I just haven't used it before.

The other question, beyond core material, is the face sheets.  A coworker suggested 18oz (oz/yd2 cloth) on the front, core, and 12oz on the back, not including an extra 12oz along the lip for extra thickness.  Not building many parts on this scale personally, that still seems like a lot of thickness (= weight).  Any other suggestions coming from experience would be greatly appreciated; please drop a line in the comments section.


Seat:

• DONE seat mold body work
• finish the seat mold
• pull a seat from the mold
• mount the seat to the nose

Nose:

• change control stick axis bolt to drilled with cotter pin (will have to remove the torque tube)
• aileron crank tube slots
• sidewalls in forward nose section (G4N13) 
• make a nose skid (G4N10)

Tail:

• AN3-26A bolt into the forward line guide bracket, replacing a rivet (G4T14)
• swage aft cables (4x remaining) 
• round off the tail skid to be more roundy than pointy

Cabanes:

• new cabane aft tubes with sleeves
• cabane compression braces (some are loose after the cabane rework from G3 to G4 design)

Wing:

• glass the ribs to LE and TE
• stiffener for aileron control line 
• leading edge shell (G4S9)

Control surfaces:

• glass the right aileron ribs
• flip fwd aileron control horn bolt to other direction ... and switch to AN3-13
• rivet aileron crank tube to trailing edge (G4S4)

Miscellaneous:

• wrap every pulley with tape and heat shrink to hold stiff and straight
• threadlock on every quick-link
• neoprene eyebolt spacer: horizontal stab (G4T8), elevator control arm (G4T16), tail strut (G4T19)
• check every quick pin for elastic retainers are both present and correct length (several are just singles)

Buy list:

• 1" wide fiberglass tape (I ran out on the aileron, also needed for the ribs to LE/TE joint)
• (1x) AN3-26A for fwd tail line guide (G4T14)
• (2x) AN3-13 for aileron fwd control linkages
• nose skid plastic plate
• covering and adhesive
• (1x) 1/8" SS quick-link