Fill & Finish Questions From TRF, Part 1

On TRF, Neil W had some questions about how I fill and finish. I tried to  answer all his questions. Neil's quotes are in italic -

I am mostly using the finishing strategy that hcmbanjo has documented on his blog. In this strategy, filler/primer is functioning primarily as filler. So you sand off just about all of it except what remains in the various cracks and indentations, and then it has done its job. The reason to remove as much of it as possible is.... well, I guess it's mostly about weight, but perhaps also a bit of thickness, although I can't attach any numbers to either of these. A good area for experimentation."

Hi Neil,
I do a two step fill. First is the CWF to fill 80% of the balsa grain and tube seams. The second step Duplicolor Filler/Primer fills any remaining grain and seams after sanding.
Removing most all of the filler/primer is not for weight reduction. I'm a sport flyer, a little extra weight doesn't bother me or my flights.
This two-step fill process is something that works for me - In my teens, I hated the four passes of brush on and sand steps using sanding sealer. I read about some multiple step process some builders employ and wonder how long it takes them to finish a rocket.
My fill and finish steps are probably quicker. What slows things down is the paint dry times between finish colors.
I have used every available filler/primer out there and have found the automotive Duplicolor Filler/Primer to be the best. Some filler/primers sand like rubber!

"In general, I'm not as obsessive about removing filler/primer as Chris is; his body tubes in particular have *very* little grey left on them when he's done with them. I tend to run out of patience before I get to that point, but my mindset has still been to remove "most" of it."

You probably don't have to take it down to the tube and balsa surface like I do, but this works for me. When you sand to surface you will see gray filler/primer left in the remaining seams and balsa grain.

"I am definitely leaning towards the idea of leaving a bit more primer on in areas where scuffing is a danger. That might help avoid problems. However, sand-through is still possible at high points. I guess if I can just minimize them, then a bit of touch-up here and there with CA is no biggie."

Occasionally I do sand too much and can scuff the surface. This shows up when I shoot the first gloss white coats. I usually do a gloss white undercoat. Take the undercoated model out in the direct sunlight and you'll see the rough spots that require smooth sanding. There are always some rough areas and glue boogers that need to be smoothed out.
If an area was rough or sanded through, I do some light sanding of those areas of the white base coat. These (sanded) rough areas disappear with the second white undercoat.

"I can't imagine leaving filler/primer unsanded. After a heavy coat, it's usually quite rough and bumpy. However, the idea of performing minimal sanding only to smooth it out definitely is something I'll explore, rather than trying to remove most of it."
"Are you talking about filler/primer or regular primer? As mentioned above, I find it hard to envision leaving filler/primer unsanded. Regular primer, though, I usually don't sand (in fact I usually apply it in the same painting session as the base coat).

The Duplicolor Filler/Primer I use drys dull and slightly rough. At a minimum you do have to lightly sand to smooth it out.
I only use the CWF first, then the Duplicolor Filler/Primer (FP101) and sand to surface. No other "Primer" after that, just the white undercoats.