My best advice is to really roughen the surface of the G-10 using 50-80 grit sand paper before the fillets are made. What is the body tube made out of and what epoxy are you using?

Did you sand and rough up the G-10 and then clean it with alcohol before attempting to glue the fins? Did the root of the fins have the same treatment?

My best advice is to really roughen the surface of the G-10 using 50-80 grit sand paper before the fillets are made. What is the body tube made out of and what epoxy are you using?

I used Devcon, high strength,30 min. set time epoxy .

My best advice is to really roughen the surface of the G-10 using 50-80 grit sand paper before the fillets are made. What is the body tube made out of and what epoxy are you using?

I used Devcon, high strength,30 min. set time epoxy .

forgot to tell you i have a fiberglass bodytube

Did you sand and rough up the G-10 and then clean it with alcohol before attempting to glue the fins? Did the root of the fins have the same treatment?

I rough up everthing and cleaned the parts with 3 parts water , 1 part 70% alcohol , and 1 drop dish soap ( mold release cleaning recipe)

I have been taught that before I start to build a glass kit, I wash everything in soapy water and rinse, then air dry to remove any mold release or anything else on the glass. Before I glue, I sand and clean the surface again with alcohol. I either use ethanol or methanol. Since this rocket has through the wall fin attachment, one of the reasons for fillets to crack would be because the fin is flexing, so the next question could be, did you get the root glued well to the motor tube, and the front and rear of the fins glued to the motor tube centering rings? If this is really secure, then the fins flexing at the outer body tube should be reduced and your fillets should not crack. Another thing might be to make sure the areas where the fillets are applied are really cleaned well also, just as you would have done for attaching the fins. Have you done everything just as the instructions described, and all the fins are in alignment? If the fins are flexing in flight, you could crack fillets, or a hard landing on the bird's side could cause the fillets to crack. Once cracked, I usually clean the area with alcohol, let air dry and then, depending on the size and width of the crack, I use CA to seal the crack, or if larger, gap filling CA, then sand as necessary and refinish. I hope this helps or provides some insight.

Some quick cure epoxies are mediocre in terms of flexibility. I use Aeropoxy from Giant Leap.

Pros:

It flows nicely.
It is elastic compared to some quick cure epoxies.
It is really strong.

Cons
It takes 4 hours to set up.
Cures in 24 hours.

I also make pretty big fillets. As pointed out, sanding is critical. Also the 0.063 fiberglass fin stock can break/flex.

Or, the easy fix once the rocket is built -- try a bigger chute. I have a Black Brant that ate fins, until I learned to just up the chute size. Slower landings = less fin damage.

Steve is right in what he is writing. He likes Aeropoxy from Giant Leap, I am hooked on the West System. They make a five minute that I use for tacking fins on to hold them in place until the hard stuff comes. Then I use the 105 epoxy in quick cure that takes up to 8 hours to dry, or you can use the slow cure that takes 24 hours. The slower the cure or longer the time, the greater the strength is in the bond. I as micro fillets (#406 - Colloidal Silica) in my epoxy when I am doing fillets. It flows nicely and they come out smooth with very little to sand and smooth. James Russell taught this too me. Mark Lionberger uses the West System also. There are 2 places to buy it in Fort Collins, 1 in Denver on the south side, and of course the mail-order method. This is really good stuff also if you are going to fiberglass or use carbon.

I've been using West Systems for over 20 years - first boats, then an airplane and extensively in rockets. I don't use it to tack things though - takes too long to cure and in anything where there is heat contact, West Systems begins to soften at 125 degrees F. For fin roots, I use JB Weld where they contact the motor mount tube or airframe in MD projects.

Fin fillets I usually mix chopped fiberglass or kevlar pulp to the West Systems, again excluding MD projects where I will use Aeropoxy or JB Weld mixed with the same. Silica or glass microballoons are great where you'll need to sand, but they add no strength and in fact significantly reduce the strength of the epoxy. Glass or Kevlar pulp strengthens fillets immensely, but also increases the difficulty sanding.

For laying up composites, I usually use 105 West Systems resin with 206 hardener although for large projects where I need significant layup time I use 209 hardener.

I've had similar experiences. I built a PML Patriot for my L1 cert, and used the 30 minute epoxy. It used Quantum Tube and 1/16" G-10, both pretty flexible. I also had problems with cracked fillets, even a chunk came off once so I had to re-do one of the fillets. The hobby epoxy seems pretty brittle. I was never able to determine whether it was fin flutter on the way up or a hard landing on the way down. I guess you could try the upgraded parachute idea now that the rocket is already built.

Since then I've used Aeropoxy and Colloidal Silica on my subsequent builds. I'm impressed with it so far, no issues yet.