Duane Allman Rainbow Aero Modelers Society
Below, I have given some recommendations for making repairs to and building model airplanes that have helped me. These were obtained primarily through trial and error and are not necessarily the only way. Other model builders will have techniques that may work just as well.
For dings I don't recommend those expensive wood fillers sold in the hobby stores. If you like them my suggestion is to use Red Devil's spackling compound which is much cheaper and as far as I can tell, about the same. My own personal preference is Ace Hardware's own brand of vinyl spackling. It is easier to use in that it is pastier and seems to stick better to wood, especially balsa. Better yet, it is even cheaper than Red Devil's Spackling.
For fillets or a tight fit between the wing and wing saddle, a hatch and the fuselage, or anywhere you want a nice close fit, my recommendation is the resins that are available from the hobby store. Resin's don't stick to wax paper so just insert a piece of wax paper between your wing and the fuselage and apply the resin to the fuselage using a putty knife.
For filler I use micro balloons mixed with Pica's "fillit". Pica's fillit looks like fine sand and makes it easier to sand down afterward. I add the micro balloons and fillet together, maybe 3 or 4 to 1 (micro balloons the most) to the resin. If you have access to lots of sawdust, give it a try.
The resin mixture requires a catalyst, which tends to thin it out slightly. Since the resin mixture "flows", make it a bit stiff. Also, you can save the resin mixture for several weeks in a closed container as long as the catalyst is not added. It may thicken a bit during storage but you can always thin it out later by adding more resin. "Bondo", available from automotive stores can also be used as a filler, but it is much harder to sand.
If you need to re-enforce the center section of a sheeted wing with fiberglass, my recommendation is to apply it with epoxy glue thinned with alcohol. DON'T USE CA! The alcohol seems to slow down the curing process with epoxy so it gives additional time to work with. Just brush the thinned epoxy on and let it cure overnight. You can lightly sand the hardened epoxy and apply a second thinned coat if you want a particularly smooth layer. Note: adhesive backed is not as easy to apply over epoxy so a little extra care may be required.
Recently, a fellow flyer asked me to repair a damaged fiberglass cowl. For about $5.00, I was able to fix one up that cost$30.45! It turned out looking almost new. First, I used gap-filling CA with some accelerator to attach broken pieces and repair some cracks. Then I lined the entire inside of the cowl with convenient strips of Bondo brand fiberglass (available from auto stores) which is quite thick, using epoxy thinned with alcohol. This gave the cowl strength. Finally, I filled the outside of the cowl with resin as described above. Sanding, with some repeated fill in low spots, did the trick. After a coat of paint, the cowl looked virtually brand new.
If you acquire a model with fiberglass cowl needing fill due to engine modification, the technique above will do the job for you.