Foam core composite motorcycle top box surface finishing advice please

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MotoFairing

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This is my first foam core composite project. An aerodynamic motorcycle top box and this is where I am up to.

The foam layers were hot wire cut, stacked glued together and sanded to shape. I used some pastor stuff to fill low spots but I should not have. I then put layers of fiberglass on, and sanded with a random orbital sander but I ran out of epoxy which explains the unfinished job. Mistakes were made.

Now I am coming back to the project as I have more epoxy but I don't want to stuff it up so I am asking for some advice.

Current plan is:
1) Complete fiber glassing
2) Fill the low spots and sand smooth
3) Bottom then top, Layup carbon fiber (400g twill). Hardening under vacuum to ensure it stays down with perforated film and bleeder cloth.
4) Cut in half
5) Remove insides and sand to the carbon fiber
6) Reinforce box on the inside to satisfactory stiffness and hinge and bracket mounting points
8) Paint

Before I go ahead I want to get some advice to hopefully avoid some more stuff ups.

If I can get this finished I won't need a backpack on my motorcycle and I can advance to an exciting new fairing project.
 

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MotoFairing

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Fiberglassing done for today. I am getting better at it.

Tomorrow filling, primer and associated sanding. I have Honey Wax mold release but it is 4 years old I wonder if it expires.

I will buy some pva release agent, primer as well as a "jenny brush" and foam surf board foam sander block.

I have been questioning method of cutting a door into the box.

After a headache I have decided to do a horizontal cut with the angle grinder to remove the mold and produce the top and bottom parts.

The lip of the bottom part will have a reinforced lip with the top part (masking taped edge) secured in place with duct tape while it hardens. Hopefully that will avoid warping and I should be able to get a tight seam.
 

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wsimpso1

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Lots of info on this forum. Here are a few That showed up with a quick stab at the search tool.





There is more… please do some searching before posting. That way you start posting from knowledge and can ask much more focused questions in your posts.

The basic process after making the fiberglass-epoxy parts for airplanes is:
  • Apply dark epoxy primer;
  • Over fill with dry micro;
  • Fair with sanding sticks and sharp 36 grit to achieve faired shape;
  • Remove scratches with 80 grit, then 120;
  • Epoxy wipe process to fill pinholes and harden the surface;
  • Sand to 220 grit;
  • Prime and block sand (and prime and block sand...);
  • Topcoat.
There is detail in each of the steps.

Billski
 
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MotoFairing

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Lots of info on this forum. Here are a few That showed up with a quick stab at the search tool.
Thanks I had a read.

Over the last couple days I prepared the plug and yesterday I laid a top and bottom layer of 400g carbon fiber twill. 0.6m x 1.0m on the bottom and 1.0m x 1.1m on the top. I trimmed a few corners to fit and stuck them on the bottom so as not to waste the material.

The twill draped well around the box unlike the plain weave fiberglass which was a real pain to work with.

I then covered with peel ply and breather cloth and vacuum bagged. Since I struggled to find all the leaks I had to keep turning the pump back on for six hours then it lost pressure as I slept.

There was some crimping which is not so good. Also I managed to leave a rectangular foam pad between the peel ply and the breather cloth whoops.

I think I will need more layers of material to increase the sturdiness of the box although I didn't add those in this lay up because it was daunting enough as it was.

I think I should get myself a pressure regulator for the pump and I will definitely do a female mold for the next project.
 

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wsimpso1

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Cool. I take it the pointy end points aft. Now you get to figure out how to make it open and close and seal against rain. Then fill and fair, then paint, or atleast clearcoat it. How do you plan to open/close/seal this?

Making a female mold from a plug is the right way to do this.

One ply of 400 g - that is 12 oz/yard. The sailplane guys use 6 pcf Divinycel foam with a ply of 6 oz (200 g) on each side and find that pretty sturdy. If you need more stiffness than one ply each side of foam, you might go with two plies of 6 oz or one 12 oz on the outside and only one 6 oz on the inside. At the bolt holes for attachment, you would omit the foam around where the holes will be and throw in a few plies overlapping the edges of the foam.

The following shop appears to have it all for making a vacuum pump rig that works.

I have used adjustable bleeds when I was working with wet lab pumps (pucka-pucka-pucka...) run continuously. They put out an oil mist. I have since bought used dry vacuum pumps with solenoids, and adjustable pressure switches. Parts from JoeWoodworker.com.

Billski
 

MotoFairing

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Yes I will do a female mold next project. The surfshop guy said there was cheaper resin I can use for mold making.

Yes pointy end goes back, acting like a tail cone.

I am thinking about the door now.

I got some pizza to ensure the door is big enough.
 

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Geraldc

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For vacuum bagging I used these bags for clothes.
vacuum cleaner gets most of the air out but a vacuum pump is better.
Screw seal and valve .
1631914266253.png
 

MotoFairing

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Yes I have used similar vacuum cleaner vacuum bags on a previous project.

I have removed the plug and now have a sense of the stiffness of the composite. When tapped with my finger I can hear the higher pitch due to the extra stiffness. The flat areas with only one layer as well as the load bearing bottom will need reinforcement.

The vacuum bagging worked well, current weight is 974g which roughly 32% being epoxy.

Next step. For the door I am considering attaching a pair of stainless steel hinges to two pieces of wood that will then be glued with micro spheres into the bottom of the box/door. The key lock will also be attached in a similar way but to the top. The door frame can then be put together for a flush fitting water tight door.

The door is fairly stiff already with 1-4 layers of carbon fiber and slightly curved edges but will be further reinforced.
 

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MotoFairing

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I have been a bit lazy but progress is being made.

Door and lock added, top and opening reinforced, box mounting holes and brackets made.

The past couple days I have been filling with micro and sanding it with 180 on the random orbital sander to get it ready for some Rust-Oleum white pearl spray paint.

It has dawned on me that I will have to look at this box every time I ride my motorcycle so I better get a decent finish on it.

Using micro is good.
 

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wsimpso1

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Orbital sanders tend to be too soft in their support of the sandpaper, allowing seams and cloth texture to "print" when you get to paint, particularly when this much of the underlying composite becomes visible.

In general, the approach is usually to work to profile with hard sticks 36-40 grit, and as dark underlying surfaces "come up through" the micro, change to 80 grit just to remove the deepest scratches, also with hard sticks. On large flat surfaces, a hard stick is really a hard stick. On a surface like yours it would be a hard foam form cut concave to a larger radius than your part. This process removes high spots, profiles the surface and is stopped before it exposes the graphite fiber.

Given where you are now, I expect that a layer of primer will reveal a disappointing field of pinholes and print through the underlying structure and cloth.

Two approaches can solve it:
  • Begin again with dry micro overfill:
    • Refill the surfaces and profile sand, stopping when the underlying stuff begins to come up through the micro;
    • Do multiple coats of epoxy wipe to fill all the pinholes - five coats, wiped on and off - letting it just start to gel before applying the next coat. This technique will fill an amazing amount of small dents, bubbles, etc;
    • Sand off resulting high spots and smooth surfaces with 120 and 180 using hard sticks;
    • High build primer and block sand, repeating as needed until it looks good enough for paint;
  • Alternative is to skip the overfill, and do the rest of the above, but with spot filling and as many cycles of high build primer and block sanding as you need to get a smooth continuous surface. Probably a lot of cycles.
My preferred approach is the first option. It takes less time and makes a nicer looking surface much easier than the second method.

Be forewarned:
  • Spot filling with dry micro leaves a slightly rough surface and usually incompletely filled dents and such - it is just too coarse by itself for small defects, but it works great for bigger spots as well as for overfill and profiling. The epoxy wipe will tend to fill small defects;
  • Dry micro/cabosil mix can be a bit of a chore to level, but you can fill dents, bubbles, and such with it after all profile sanding and before the epoxy wipe. Be sure to scrape it from the surface before you move to the next spot or you will end up with a high spot there;
  • Spot filling with phenolic balloons will tend print on hot or cold days (high thermal expansion coefficient makes them high on hot days, low on cold ones) - I do not use this stuff on parts anymore, only on tooling to be used in managed temperature shop space;
  • Spot filling after epoxy wipe will demand a local repeat of epoxy wipe, so do as good a job as you can before epoxy wipe;
  • Lacquer and polyester spot putties tend to shrink over time and show up as low spots later.
The methods detailed in curedcomposites.com really work well.

Bill
 
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MotoFairing

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Project finished.

Painted the box and painted some more then clear coat.

Added a reinforcing rib to the bottom of the box to reduce flex at the mounting points.

The locking mechanism broke because micro got into it so I replaced it.

Project finished.
 

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