I just got a U-Can-DO 3D, and I am putting an OS .61 FX in it. I know that I should add some tri. stock to the firewall. I also heard that I should strengthen the fuse., but I dont know what to use, or how to do it.

Thanks

Epoxy the tri stock to the firewall and fuselage sides. The stock firewall is butt glued to the sides with hot glue.

For the fuselage, look where the main wing goes into the fuselage. You will see how the sides of the fuselage are cut out for the wing. On a "not so perfect" landing this area tends to crack. On the inside, cut some 1/16" ply to fit the area and laminate in place with a THIN coat of epoxy - clamp as needed until dry, OR cut and fit some 3/32" balsa and laminate in with CA.

These are the two best known problem areas for the U-Can-Do

First of all are you good at landing? If you land 'softly' 90% of the time then you do not need to strengthen the Fuse.. The planes fuse is only "weak" when it's on the ground..
The one thing on the fuse to worry about is the landing gear! it's really weak. all you need to do is lay some Fiberglass over it and Coat THE WHOLE LANDING GEAR AREA WITH EPOXY! Also it wouldn't hurt to lay a layer of epoxy in the tank compartment for fuel proofing purposes.

The firewall is a different story. See if yopu can't pop it off the fuse and sand the contact area's down... then EPOXY it in there! then use the tri-stock
TIP: it's easier to install the tristock while the Firewall is off.
Now you COULD pin the firewall using toothpicks... Not necessary but good peice of mind..

epoxy the wing bolt plate in, lost my first ucando early in its life due to that little piece of wood coming unglued while in flight... the wing survived, and that is about it. (the wing took almost two minutes to come down! it just 'rolled' down...)

Consider GreatPlanes Milled fiberglass to mix in with the epoxy. Sort of eliminates the use of cloth. Works real good.