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Primer, filler, fiberglass, and plastic questions

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grandcross

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So I'm at a point where it's time to start adding details to my fiberglass airframe for my Saturn IB build (https://www.rocketryforum.com/showthread.php?135578-Another-Saturn-IB), and I have some choices to make. So as with all choices of this type, I ask the hive mind...

The body tube is home made fiberglass. The details will be styrene (for the ribbing, etc...) and PLA 3D printed parts (ullage motors, APS, antennae, etc...). So my questions fall into 3 categories.

1. Priming

The fiberglass will need it. It's a fairly coarse weave, so there may or may not be a bondo requirement similar to the Polecat tubes. I plan on testing my primer for this soon. So should I do this before adding the details?

If I do it first, then I have to worry about whether the glue I use will stick to the primed surface (more on that shortly). Then I may need to prime again once the details are added, potentially with a different type of primer. It would definitely make priming the FG easier with far fewer nooks and crannies to worry about.

If I do it after, I'm bonding straight to the FG. Downside is more nooks and crannies to sand, and the sealer may not be appropriate to use on plastic parts.

2. Primer

Next is what type of primer should I use? Typically I use the gray Rustoleum primer and have had great results with fiberglass. Has anyone used their plastic primers such as this or this with:
- Styrene
- PLA
- ABS
- Other plastics like PETG?

Would spraying the plastic primer over the regular primer cause any problems? Would it be required?

3. Glue

Options seem to be thick CA, thin CA, or some other CA variant, and epoxy (probably rocketpoxy?) This would be for styrene (fairly thick 0.06", so CA melting doesn't seem a serious concern. I already tried some on a scrap piece) and the PLA 3D printed pieces to the fiberglass or primed fiberglass. Opinions?

Remember that this is a HPR so there will be some flight stresses. For some printed pieces I added a post that will go through the wall to help keep it place. Adding a backing is probably not an option at this point as parts of the air frame are already sealed.

So let's start the discussion with this. I may have more questions or follow ups later :)
 

mccordmw

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If you have coarse weave to fill all over your tube, you should consider skimming with laminating epoxy like the boat makers do. You'll end up with a good epoxy surface to sand and attach to with your details.

[video=youtube;cWSP7YCLu1I]https://www.youtube.com/watch?v=cWSP7YCLu1I[/video]

[video=youtube;eO8sEjt4rOw]https://www.youtube.com/watch?v=eO8sEjt4rOw[/video]
 

grandcross

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If you have coarse weave to fill all over your tube, you should consider skimming with laminating epoxy like the boat makers do. You'll end up with a good epoxy surface to sand and attach to with your details.

[video=youtube;cWSP7YCLu1I]https://www.youtube.com/watch?v=cWSP7YCLu1I[/video]

[video=youtube;eO8sEjt4rOw]https://www.youtube.com/watch?v=eO8sEjt4rOw[/video]
Which would also make for a very heavy rocket!

I tried some primer on a piece of fiberglass trimmed from the main body tube. It primed quite nicely, so I don't think the cloth coarseness is as bad as I thought. Overthinking things I think.

So the big questions still are: Prime before or after? Best glue?
 

mccordmw

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Which would also make for a very heavy rocket!

I tried some primer on a piece of fiberglass trimmed from the main body tube. It primed quite nicely, so I don't think the cloth coarseness is as bad as I thought. Overthinking things I think.

So the big questions still are: Prime before or after? Best glue?
Actually, no. If you look at him. He's adding only about 6 ounces of epoxy in two 3 ounce pours. It's troweled on very thin and allowed to settle.

If you want to go the primer route, I'd glue first. You want the glue to bind to the toothy surface of the rough fiberglass surface for a good mechanical bond.
 
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