Newb question about fiberglass

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

PropellantHead

Well-Known Member
Joined
Mar 15, 2010
Messages
561
Reaction score
8
I'm building my first fiberglass kit and, of course, I need to score all mating surfaces to ensure a good bond with the epoxy, but how much scoring is needed? I attacked my switchband and coupler first by hand with some 80 grit and, dang!, this stuff is hard to make a dent in!

I can definitely feel that it's a rougher surface now by touch, but visually, you can't really see any difference in the sanded vs. unsanded areas unless you've got a bright light on it.

Any thoughts on what optimally prepared surfaces look/feel like? Or, better yet, can anyone share a pic of one?

(and, yes, I did try to search for previous threads with similar info but struck out)
 
80 grit is fine - nice dull finish then alcohol area prior to epoxy. Here a couple pics of what your looking for - easy peasy.

IMG_9634.JPG
IMG_9455.JPG
 
Even more important, make sure you WASH ALL FG PARTS before bonding. Out of the box, they are covered in chemicals used during the manufacturing process that will get between the FG and the epoxy. I use warm water and dish soap.
 
I attacked my switchband and coupler first by hand with some 80 grit and, dang!, this stuff is hard to make a dent in!

I cheat on this.. I have a benchtop harbor freight belt sander. Makes short work of any FG sanding job as long as you don't care if it looks nice when you're done :)
 
Also, you don't have to use sandpaper- sometimes using a metal file works better/faster.
 
You should check out the top sticky in the high power forum "Properly bonding composites and what your government doesn't want you to know." it has a lot of good info about this and other relevant info.
 
You should check out the top sticky in the high power forum "Properly bonding composites and what your government doesn't want you to know." it has a lot of good info about this and other relevant info.
+1. This has all the answers to get you going the right way.
 
Even more important, make sure you WASH ALL FG PARTS before bonding. Out of the box, they are covered in chemicals used during the manufacturing process that will get between the FG and the epoxy. I use warm water and dish soap.

+1 on this...I almost took my Adventurer 3 in the shower with me, but knew my wife would have had a field day with that!
 
80 grit is fine - nice dull finish then alcohol area prior to epoxy. Here a couple pics of what your looking for - easy peasy.

View attachment 323353
View attachment 323354

Ok, it looks like I'm doing ok. My parts looked like that after sanding (see pic) but after I washed it all off, you couldn't visually tell anything had been done. That's when I started doubting myself and wondering whether I went deep enough, and if I sanded deeper should I worry about weakening the FG appreciably, etc... Normally I wouldn't fret about it this much but this is my L3 cert project and I really want it to be as perfect a build as possible. That way, when it crashes and burns later, I can at least say it wasn't due to a problem with my construction techniques. ;)

Thanks for the pics!

well, nevermind about the pic...the forum software isn't allowing me to upload and tinypic.com seems to be down at the moment. I may try again later.
 
Even more important, make sure you WASH ALL FG PARTS before bonding. Out of the box, they are covered in chemicals used during the manufacturing process that will get between the FG and the epoxy. I use warm water and dish soap.

Oh, yes, I've been diligent about this. I washed all the parts down with a 3:1 water/alcohol mixture with a tiny bit of dish soap thrown in BEFORE sanding (it was caked with epoxy / FB dust from the manufacturing & cutting processes) and then I thoroughly cleaned them again afterwards. And I have 20+ itchy fiberglass splinters to prove it. ;)

Note to self: Nitrile gloves are NOT adequate protection.
 
You should check out the top sticky in the high power forum "Properly bonding composites and what your government doesn't want you to know." it has a lot of good info about this and other relevant info.

Noted! Will do!
 
Ok, it looks like I'm doing ok. My parts looked like that after sanding (see pic) but after I washed it all off, you couldn't visually tell anything had been done. That's when I started doubting myself and wondering whether I went deep enough, and if I sanded deeper should I worry about weakening the FG appreciably, etc... Normally I wouldn't fret about it this much but this is my L3 cert project and I really want it to be as perfect a build as possible. That way, when it crashes and burns later, I can at least say it wasn't due to a problem with my construction techniques. ;)

Thanks for the pics!

well, nevermind about the pic...the forum software isn't allowing me to upload and tinypic.com seems to be down at the moment. I may try again later.

If your parts looked like that before you cleaned them again, you're golden. All you did was wipe away the dust particles in the grooves from sanding. The grooves are still there, the surface is still roughed up.
Here's the basic idea and debate: the epoxy settles into the sanded marks, thereby creating a miniscule mechanical bond with the surface. Now, the debate is whether or not that mechanical bond it's actually necessary because the epoxy can actually form a chemical bond with the fiberglass if prepared properly (rubbed just before bonding to "excite" the molecules).
The good news is that the sanding prep followed by cleaning the dust has been proven to work thousands upon thousands of times with successful flights. Regardless of the science behind which method works, field tested experience means that it does work, period.
So, again, if your parts looked like that picture before a final wipe/cleaning, then you're don't what most of us do, and your rocket will have been built with the bonding technique proven by experienced flyers.
 
Even more important, make sure you WASH ALL FG PARTS before bonding.
+1 on this, and do it before putting any tools on the parts that can carry contaminants to other pieces. I usually wash the entire kit before I get going (talking fiberglass kits here, not plywood :wink:). No use having a clean part and then hitting it with a dirty file.

As for washing, I use about 30% metho (or IPA) and a couple of drops of dishwashing detergent in a 1.25 liter bottle. I also wipe each joint location with MIBK (others use acetone or other solvents) just prior to bonding, just to get it "squeaky clean".
 
+1 on this...I almost took my Adventurer 3 in the shower with me, but knew my wife would have had a field day with that!

LOL! Note to self...don't take the new Predator in the shower-you WILL hear about it! I already hear enuff about symbolism to last me a life time!
 
One other thing to mention. I wash my parts first, especially if I've been touching them. IPA with dishwashing soap does a really good job.

From that point on it's important to have no skin contact with the parts. Before sanding the surface with 80 grit, glove up and stay gloved even during the sanding process. If you don't, the sanding can grind the oils into the surface and remove some strength from the bond. It may not be critical for most rockets, but get in the habit now if you want to push the performance envelope.
 
80 grit is fine - nice dull finish then alcohol area prior to epoxy. Here a couple pics of what your looking for - easy peasy.

I would not recommend alcohol to clean the surface, it does leave a residue.
When I laid glass in repairing boats and Vett's, Lacquer Thinner was the cleaner of choice.
It evaporates fast, completely and any residual dust can be tack ragged off.
High Quality Mineral Spirits can also be used, but on fiberglass it takes much longer to evaporate.
All cleaners will wick in at a microscopic level, so the faster the cleaner flashes off, the sooner it becomes workable.
 
Ok, maybe I'm just lucky or something. Gloves? Never touch parts? Wash parts? I have never washed anything, but I do sand before epoxy and paint. I just started using gloves just for ease of cleanup. Never been worried about contamination. Remember, my way isn't necessarily the best way for others.
 
Ok, maybe I'm just lucky or something. Gloves? Never touch parts? Wash parts? I have never washed anything, but I do sand before epoxy and paint. I just started using gloves just for ease of cleanup. Never been worried about contamination. Remember, my way isn't necessarily the best way for others.

Definitely lucky. My last FG kit was so slimy and dirty that it would never have held glue.
 
I would not recommend alcohol to clean the surface, it does leave a residue.
When I laid glass in repairing boats and Vett's, Lacquer Thinner was the cleaner of choice.
It evaporates fast, completely and any residual dust can be tack ragged off.
High Quality Mineral Spirits can also be used, but on fiberglass it takes much longer to evaporate.
All cleaners will wick in at a microscopic level, so the faster the cleaner flashes off, the sooner it becomes workable.

The water/soap/alcohol mixture was the manufacturers recommendation for the initial, pre-sanding cleaning. After sanding, they didn't seem to mention anything specific but I plan to use straight acetone. It leaves even less residue than lacquer thinner.
 
The new Tripoli Report published yesterday has a very good excerpt from the West Systems Epoxy manual. It's on the Tripoli website now for members.


Steve Shannon
 
Back
Top