I decided to start cutting out the openings for the variable inlet doors, radiator cooler, oil cooler and tail wheel first before adding any filler to the over-sanded portion of the fuselage. This was done primarily so that I could see exactly the wooden structure underneath the infilled balsa and to determine the best approach to cutting without too much further damage. The location of the fuselage formers also played an important role here in helping me determine where to cut and how much pressure to apply to the cutting blade.
The other reason is that I knew that I would be adding filler material anyway after cutting and it would appear somewhat redundant for me to do the same task twice.
The first photo was (obviously) staged to show my initial cut for the radiator inlet.
The second picture shows the rough cuts. Ugh, pretty rough I'd say so myself. Not to worry.... I hope
The third photo shows the application of Spackling Compound as filler material. Here in the 'States, we have what is commonly called "Spackling Compound" and its primarily used mostly for filling in between cracks, holes, dents and drywalls inside homes. Because it's easier to sand than the surrounding soft balsa, it serves its purpose quite admirably in preventing me from accidentally sanding too far into the wood.
Only one major drawback in using this filler. It can be applied inside an enclosed room with no problem whatsoever. However, the sanding must absolutely be done in a well ventilated area. The fine dust from sanding can raise havoc on one's lungs and can also irritate the skin (Ask me). For this reason, because I'm working inside an enclosed area during the colder season, I find myself sanding with a nice cooling fan turned on and blowing the dust away from me. Otherwise, I would gladly sand outside with a nice gentle breeze while enjoying the scenery.
When carefully sanded using progressively finer grades of sand paper, this compound really ends up as an extremely smooth finish and ready for primer. I start out using say # 360 grade sand paper -no rougher than that, - as mentioned, the compound sands quite easily. I use this medium course sand paper to gently sand away most of the extraneous material and to help shape the final results as I get closer to the finish.
The second grade of sand paper is usually # 400 and this is sometimes used as wet-n-dry. I decided not to use it mixed with water as: 1) the compound is water soluble and 2) the water will also affect the surrounding wood. This sand paper is used to help finalize the smoothness of the overall finish and to remove any small particulate material remaining that otherwise would "pull" away some of the surrounding material if sanded with a more course sand paper.
The third (and for me) last sand paper is # 600. This is just to smooth out the surface and prepare it for primer paint. I could use a higher grade (more finer) type of sand paper, but for me, these three grades at this point are sufficient. When sanding the dried primer, that's when I'll probably use a higher grade.