PC-7 epoxy putty- has anyone tried it?

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I am pretty sure I've tried that brand. I find hardware store epoxy putties too hard to sand. Maybe you could try mixing in some microballoons. I personally use Superfil. Microballoons are pre-mixed in there.
PC-7 is gritty and will leave a rough surface to fillets, and it is hard to spread evenly - although I have not blasted it with a heat gun while trying to draw a fillet. It probably has some applications, but I think you are better served by a number of other products for fillets. However, if you are going to cover it with tip-to-tip glass or carbon, the gritty surface would probably help with adhesion.
I'd be a little concerned about the strength.

  • Tensile Shear Strength
  • ASTM D 1002
  • 2150 PSI

  • Compressive Yield
  • ASTM D 695
  • 4245 PSI

  • Flexural Strength
  • ASTM C 580
  • 2920 PSI

  • Water Absorption
  • ASTM C 413
  • 0.44%

  • Weight(Specific Gravity)
  • ASTM D 792
  • 1.16g/cm³ .671oz/in³

  • Hardness Shored
  • ASTM D 2240
  • 76 (1wk)

  • Heat Deflection
  • ASTM D 648
  • 120°F

Now compare that to Rocketpoxy.

Shore “D” hardness 85 ASTM D2240
Tensile strength 7,600 psi ASTM D638
Compression strength 14,800 psi ASTM D695
Typical operating temperature -50°F to 175°F
Maximum use temperature 225°F (107°C)
It works great , you can smooth the fillets out with alcohol and your finger once it starts to setup. I used it on my l1 and l2 rocket have close to 20 flights on it no issues .

That said I now use rocketpoxy .