Sorry, Dazza98. Didn't explain very well. Hope these pictures do a better job.
Here we go with the interior fin fillets (fin to motor tube/CRs).
The plan is to use a syringe to inject fillets thru the airframe holes into the valley where the fin joins the motor tube.
The CRs at each end are the dam that holds the fillets in.
So the idea is to inject the epoxy/CF mixture into the holes and then tilt the airframe up and down so the epoxy flows evenly along the valley.
Then use a pair of spirit levels to make sure the airframe is level while the epoxy cures.
There are 6 fillets: do first 2, let cure then next 2, let cure then last 2.
Set up the airframe on a level workhorse. You can see the holes to do 2 fillets
Mix the epoxy and load with chopped carbon fibre (CF).
Inject via syringe into the holes and tilt airframe back and forth to let it run evenly along the fin/mmt valley
Then level airframe for fillets to cure
The other bonus in using the CF it that it is easy to see the epoxy thru the fibreglass airframe.
Here you can see the 2 fillets looking from the inside thru the the motor tube.
The epoxy has spread fairly evenly along the whole length of the fin/mmt joints.
Here is the same 2 internal fillets looking from the outside thru the airframe.
The CF is amazing stuff. Only takes a little bit to really load up the epoxy.
The first batch I put too much CF and and it was too thick for the syringe.
Injecting the first 2 fillets was pretty scary.
I think my mixture was still a little too thick and I was worried it wouldnt flow along the whole length.
But it eventually did.
Second 2 fillets went much better: got the mixture right and it flowed easily.
Used about 30mL of epoxy/CF on each fillet for a total of 180mL.
So that's the interior fin fillets done.
Pretty happy with the way the fillets turned out in the end after a shaky start.
As long as you have access to the CF its a pretty simple and fast method.
Only thing I didn't like about this method was lack of control over fillet quality.
You only get 1 shot at doing the fillets. If something goes wrong its not easy to fix.
Next is the exterior fin fillets...