Do you really need to scuff-sand prior to applying epoxy?

I had almost a full pump of West 105/205 left over after laying fillets, so I decided to do a little test. This is a WM 2.6" nose cone and a bit of nylon paracord. I did no prep work on the NC aside from washing it about a month prior. I poured the leftover wet epoxy into the NC and sunk the nylon in place. Let it cure overnight, then the next day (about 24 hours later) gave it a quick tug. This is the result of second tug.

Short answer to the question in the title? Yes.


My intent here is to give some visual evidence to help reinforce the need to sand prior to using epoxy in order to create a good mechanical bond. I observe no deficiency in performance by the WM NC. I expect that the NC of any manufacturer would perform the same when not properly prepared for epoxy.

Wow! Thanks for the visual reminder.

Yes, 'WOW'!

I discovered this on two previous fiberglass builds:

1) You really really really need to scuff it up. An aggressive and coarse paper is needed to really dig in and make pits / scratches for the epoxy to bite into / hold onto. A smear of Bondo afterwards to fill in left over scratches, dents, pits, etc..

2) Use a 'good' epoxy, like West Systems I originally had used the 1:1 'hobby stuff' and it peeled off pretty easily. I also learned about #1 at the same time! The 5:1 mix ratio epoxy is better because of less fillers, and the longer cure time allows it to flow into the cracks & crannies to make a solid 'hold'. It is usually also runnier, so it further penetrates into cracks, gaps, voids, scratches, etc..

I am curious as to how the foaming 'polyurethane' glue is on fiberglass. I know it's better on plastics, for securing nose weight in nose cones..

That's a scary reminder!

Next question: I recall some debate on the sticky thread about whether it is important that the sanding be done immediately prior to bonding, as well as some debate about whether a coarse (60 or lower) or less coarse (80-120) is best. I've been toying in my head how to measure this, but haven't had time (or a good setup) to do it rigorously. The hard part is that none of these followup questions are going to cause a truly terrible bond, so you need a pretty tight setup for controlling for things like bond size, as well as a test stand that can exert a lot of force. The best idea I had was to take ruler-shaped rectangles of fiberglass, mask off all but say the last 1/4 inch, bond them by overlapping the exposed ends and then apply a load to the end of one while the other is clamped tightly. Once my toddler grows up I'll have time to do this myself with a bunch of samples

In addition to sanding with 60-80 grit paper, you need to wash the glass to remove any mold release.

timbucktoo said:

In addition to sanding with 60-80 grit paper, you need to wash the glass to remove any mold release.

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Yup of course. That might even be more important than sanding.

timbucktoo said:

In addition to sanding with 60-80 grit paper, you need to wash the glass to remove any mold release.

Click to expand...

X2! Thoroughly wash the inside with soap and water, or an appropriate degreaser, using a water bottle brush.

When it comes to sanding I try to sand perpendicular to the forces. In this case I would sand groves using my dremel with my 60grit sanding wheel on the end of my snake and I would put deep horizontal groves, perpendicular to the direction of the force, inside the nosecone for the epoxy to cure and bite into.

Good luck.

There used to be an epoxy product called PowerPoxy Weld though heavy it could be hit with a heat gun to get it to reflow at least 2 or 3 times to "adjust" a
fin fillet. The gist of this is when I did a post-mortem on a fincan of a 4 inch cardboard and plywood rocket, I could NOT snap the fillet off the fin with my fingers.
I had to use a chisel to get it off the plywood fin. Me thinks the heat and reflowing allowed the epoxy to soak into the plywood better after scuff sanding with the
lowest grit sandpaper I could find.

Be aware with some epoxies if you hit them with the heat gun they don't reflow and just setup "right now". The PowerPoxy was the best for heat gun reflowing and stuff like Proline 4500 generally sets up quicker without too much reflowing. Kurt

I would also recommend using an amendment to the epoxy. Chopped glass, chopped carbon fiber, or Kevlar pulp will do a lot to make the epoxy less brittle. This combined with aggressive sanding-scratching the surface should give the best results.

Epoxy can be thinned by heating, but it's cure time is greatly reduced. Others have added 'alcohol' (Isopropyl, not the drinking kind) to make it runnier. But adding anything add 'something' to the mix, and reduces adhesion, strength, cure time, etc..

Of course, the geometry of an epoxy plug in a nose cone doesn't' help. In this case, a cone within a cone, the forces pulling against it immediately detach. Not like a CR in a tube, where there is a sliding / shearing force, and the epoxy 'figure' are in close contact the length of the pull. For the NC plug, a mechanical cross member thru the NC itself would have greatly helped. I would think in this case, as well, a lot of tiny holes drilled in the NC prior to pouring in the epoxy would have helped the bond.

MarineTex. Reflows with heat. Smooth as butter. Expensive like anything boat related.

I ran into this issue with my last build. I had a CR epoxied into a nosecone for an AV bay, and I guess I did not scuff it enough prior to epoxy. After the landing of the first launch it pulled free. Got lucky I guess.
Without trying to hijack the thread, has anyone tried to use PVC cleaner on plastic nosecones after scuffing. Does it work or does it soften the area too much?
~Keith

For that warm fuzzy feeling I drill a cross hole through the tip for a nail or screw and make sure it captures the shock cord anchor and then encapsulate in epoxy.

Threemorewishes said:

For that warm fuzzy feeling I drill a cross hole through the tip for a nail or screw and make sure it captures the shock cord anchor and then encapsulate in epoxy.

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I generally do similar, especially with plastic cones, but use at least 2ea 1/4" wood dowels placed at an angle from one another...

Threemorewishes said:

For that warm fuzzy feeling I drill a cross hole through the tip for a nail or screw and make sure it captures the shock cord anchor and then encapsulate in epoxy.

Click to expand...

A few people suggested some sort of cross-member. I've never done one but I agree with the concept and its added strength. I'd only add that it is in compliment to preparing the nose cone for epoxy. Based on my original test, if I had used a brass rod as a cross-member then it would in essence be the ONLY thing holding that epoxy plug in the NC.

Crash-n-Burn said:

...I'd only add that it is in compliment to preparing the nose cone for epoxy. Based on my original test, if I had used a brass rod as a cross-member then it would in essence be the ONLY thing holding that epoxy plug in the NC.

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The cross member is more than enough to hold the shock cord anchor in the epoxy plug. I have done this on nose cones of various materials with little (minor scuffing) to no interior surface preparation. The mechanical restraint of the cross member never lets the shock cord pull on the epoxy plug enough to break what ever bond if any has developed.

There is a very detailed treatise on this subject by TRF member flynfrog that should answer this question definitively.

Believe the thread is in the Techniques section.

EDIT: Strike that. It's a sticky post in High Power. Don't have time ATM to provide link.