What is best filler for epoxy bonding?

For high-strength bonding West Systems recommend '404' - High Density filler. Its a white powder, no idea what its composed of.

People seem keen on various types of finely chopped composite fibres, I bought 404 based on knowing it should work with West Systems and it being recommended by John Coker.

For tight-fitting boding such as bonding bolts to bulkheads, I use a combination of West Systems 404 "high density filler" and "406 colloidal silica". The 406 reduces the "sag", and I use it in almost every epoxy application except lamination. For looser-fitting boding such as filling a gap or the first layer of a fillet, I use chopped fiberglass rather than the

404 to give more "structure" to the epoxy.

If John Coker recommends it, I'll probably go that route. It is also easier to get on a weekend if I decide to start epoxying today.

I would have to wait until Monday to find milled fiberglass or similiar.

Brian Elfert

The page where he mentions this is:

"paint some unthickened epoxy on the wood parts before adding the filler. For example, I use West System products, so I mix up an ounce of 105/205 and stir it thoroughly. Then paint a little on the CRs at the joint with the tube. (Try not to get it on the outside of the CRs.) Then, thicken the epoxy with your chosen filler (I use 404 for most bonding) and apply your fillet as usual."

He also mentions using it throughout his L3 project.

I bought some '403' (IIRC) 'microfibres' too, they make a more sandable mix (if you are then doing external fillets). However I don't think they're the best for this, I'll be looking for something more suitable in future like 'microballoons'

If you look through his website he mentions using most of the '40X' fillers at some point in fact!

Use milled fiber... it will thicken the resin into a dense sticky paste that's good gor structural gluing.

-dave w

Is there a TAP Plastics store in your area? They're generally open Saturdays, and carry various resins and fillers. (From the description, I wouldn't be surprised if that's what the "West 404" actually is...)

-dave w

I'm pretty sure that West 404 'High Density Filler' isn't milled fibre, if that's what you mean.

Its a speckled powder with a consistency like heavy talcum powder, it doesn't get airborne easily like flour though.

But yes, West '404' is a filler intended for use with (West Systems of cours ;) ) epoxy for 'high strength bonding' and 'gap filling'.

If you are bonding fins to a motor mount and dont want them to go anywhere, I usually mix up my epoxy with some kevlar pulp from

This also works well for bonding anything in the motor mount area, or other places that high stress is anticipated, however, I dont like to use it for external filets because it is VERY hard to sand. This stuff makes a rock solid joint and is great in high heat aplications. I also put 2 layers of 6oz glass over that when it's dried. I used this method a few times and I have not broken a fin yet.

Tom Sak

This seems like extreme overkill for mounting fins.

The only time I lost all three fins (on an O motor), they snapped at the airframe. The rest of the fin was/is still firmly attached to the motor mount.

Brian Elfert

Fin bonds are seldom stressed by the average rocket motor. Strong fin bonds are very useful on a hard landing, though, especially if the fins extend beyond the aft of the rocket. (of course, then the fins just break above the fillet).

One of the reasons to argue AGAINST TTW/TTM fins on a rocket like a LOC IV: My LOC-IV has taken some hard landings, and I think I've ended up re-gluing every one of the fins on the rocket. It's a LOT easier to repair than a shattered fin near the root. I'd rather have "pop-off" fins.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Gluing fins on the tube is okay for the small rockets, but when you get into M/N/O and above, TTW is basically a requirement. Otherwise, the fins will just peel off the tube during flight.

I do have a friend who flies O and P motor rockets with durface mounted fins, but he has spent many hours engineering and attaching the fins. His fins are very small with a long root so they will withstand all the thrust.

Brian Eflert

Odd, what are you using for a chute? My LOC IV has only cracked a fillet. and I believe that was when the shock cord mount disconnected from the body tube on it's first flight.

pics and video here

Dunno about everyone else but I'm pretty damn sure West 404 isn't chopped 'glass.

If anyone *knows* better please correct me.

I just bought a quart of milled fiberglass. That is some damn heavy stuff. I'm hoping it doesn't increase the weight too badly. I really need to stay below 75 lbs for my rocket. Every little bit of weight adds up.

Brian Elfert

The rocket is pretty much stock from late 1988, with the exception of threaded brass inserts to hold the Kaplow Klips. Many of its flights were cold windy days at Danville. At least one crossed state lines.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!