It's been a couple years since I built a plane.
I've been using light weight spackle for many years with good results and it has always been easy to sand.
I just bought a small "tub" of DAP Fast & Final and after 24 hours of drying in a warm room it sands like cheese being grated.
I think I used to use a different brand..but the problem is a tub of this stuff lasts many years and I don't recall what brand "the good stuff" was.
There IS another light weight spackle called "One Time" and maybe this is the good stuff that I am trying to replace...?

I have used the Elmer's carpenter's wood filler.  It might be a bit heavy but it sands easily and I have not had any problems with dope attacking it.

I have used the Elmer's carpenter's wood filler.  It might be a bit heavy but it sands easily and I have not had any problems with dope attacking it.

Thanks for the tip Jim.
I never would have guessed that any wood dough would sand at the same rate as balsa.

   Do a search on here for the horror stories people have posted over the years about trying to paint over vinyl spackling spackling compound. It won't accept all paints and primers, especially if trying to use it for fillets. What ever you try, test it it first.
  Type at you later,
   Dan McEntee

   Do a search on here for the horror stories people have posted over the years about trying to paint over vinyl spackling spackling compound. It won't accept all paints and primers, especially if trying to use it for fillets. What ever you try, test it it first.
  Type at you later,
   Dan McEntee

 I've used the white DAP stuff for years, works great and is way better than other products I've tried that are supposedly developed for our purpose. It's rare, (usually only if it's sat for a LONG time since last used) but if needed it thins very easily with just a few drops of water. Also, it weighs practically nothing. Keep in mind though that you're only using it as a FILLER, it's not strong enough for anything else.
 
 The "trick" is that you definitely want silkspan over it before the final finish steps begin. Just keep your finishing steps in a proper order, with all of your necessary (DAP) filling and feather sanding done on the bare balsa beforehand as typical. Then, when ready, carefully apply a layer of silkspan with dope as usual. From there, carry on with your normal final finishing routines.

 Among other areas that sometimes require minor filling this is one where I always use DAP, shown and explained in Reply #100 in the below thread. Reply #116 shows another useful example.
 
 https://stunthanger.com/smf/building-techniques/hutch's-p-38-lightning/100/
 

It's been a couple years since I built a plane.
I've been using light weight spackle for many years with good results and it has always been easy to sand.
I just bought a small "tub" of DAP Fast & Final and after 24 hours of drying in a warm room it sands like cheese being grated.
I think I used to use a different brand..but the problem is a tub of this stuff lasts many years and I don't recall what brand "the good stuff" was.
There IS another light weight spackle called "One Time" and maybe this is the good stuff that I am trying to replace...?

They changed it.
Sherwin Williams has some stuff that works.
Carton has a green label. It's called
Shrink-Free Spackle.
Dries hard and feathers. The DAP didn't do either. Didn't even stick.
As others have recommended don't use it for fillets. Paint doesn't stay down there.

   Do a search on here for the horror stories people have posted over the years about trying to paint over vinyl spackling spackling compound. It won't accept all paints and primers, especially if trying to use it for fillets. What ever you try, test it it first.
  Type at you later,
   Dan McEntee

Dan...I don't have much experience with tissue and dope...but I plan on putting tissue on all the concave areas where the biggest build ups of spackle are going to be. I am just after the finish that looks OK from 30 feet..but I want durability and some level of basic decency...
Ken....
My Light Weight Spackle Drama has been solved.
I ordered a different product called RED DEVIL ONE TIME.
I'm 99% sure this is the stuff that I've always used and it sands / feathers out very nicely.
The DAP product behaves like parmesan cheese being shredded.......
I thought  I just got a bad batch...but you had the same exerience with DAP.
Modeling supplies are a some what TOPSY_TURVY / FICKLE BUSINESS..but I've only taken a 2 year break from it and it's been kind of like THE TWILIGHT ZONE getting back to it.....
 

I am taking back all of the good things I said about DAP.  It has changed big time.  Way too soft.  The Sherman Williams seems better but still not as firm as I like.  I bought a small tub of MinWax wood filler.  It is not as light as the Lightweight DAP but it sands and feathers like balsa.  It starts out a dark redish brown then has the same color as balsa when dry.  A bit pricy but what isn't these days.

Ken

Thanks for the tip..I'll give the Minwax wood filler a try.
Pricey....it depends
There are times when the most expensive way to do this hobby is when we try to do it cheaply....

I remember Windy saying Grey Acrylic lacquer primer sanded with 80 grit sandpaper is a good way to keep dope from lifting off fillets, that and keeping the paint to a minimum. He also said the best fillets are no fillets at all. Make the fillets on the inside of the fuse with strips of triangular balsa.

Kevin...I never would have guessed that dope lifting away from a spackle fillet would be a problem.
If I wasn't doing a 1/2A C/L plane I'd most likely do the fillets with Bondo. ...and with a balsa tri-stock base not much bondo would be needed  for my 1/2A project.
It sounds like Windy was quite the Experimenter who tried many things....lacquer primer is compatable with dope which is good to know.

Is there any other way?  And here I thought I was the only person left in the PA world still using Balsa fillets!

Ken

No, you are not.  I have been using balsa fillets for a long time.  In areas with too much curvature, I will cut a slice to match, like a thick rib.  This adds strength to the fuselage-wing joint and also to the stab-fuselage joint.  I sand to contour using a small piece of tubing wrapped with sandpaper.  Clean up the edges and blend in with SuperFil.

theres a 2 part primer that uses a catalist and sands easy and not as heavy as bondo...might work good for filliets

How about fine balsa saw dust mixed with just enough easy sanding resin...?

Years ago I was in an "experimental mood" at a time when I had a bunch of douglas fir [lumber] saw dust and a bunch of polyester resin that was going bad. I mixed the 2 together and noticed that....

the wood got darker..about the same shade as your basic, standard grade  "dog do"
the overall appearance was that of "dog do" ..[enough to fool anyone]..so it really didn't take much artistic talent to create a few "exact scale replicas" of the real thing.
The family owned gym I belonged to always kept their floors very clean and some areas were carpeted.
They also owned a bull mastiff...[but it was never allowed in the gym].
You should have seen the look on the owners face when she first laid eyes on the very realistic looking "surprise" I steathily placed on her floor .....

I'm experimenting with lightweight spackle with about 25% Durhams Rock Hard Water putty added.  Add a touch of EZ-Kote as a binder/thinner.  Mix to the consistency of toothpaste.  So far so good.  Very easy to sand.

I've seen Durham's Rock Hard on the shelves but never tried it.

A combo that works to seal the opening in the bladder compartment of combat planes is spackle mixed with 5 minute epoxy. It creates a nice smooth seal between the foam wing, the plastic covering and the fuel compartment.
Now I'm wondering why I never tried it for a small fillet.

Spit and an Iron is the best method to take out dings from balsa. But who wants to wait 3 minutes for a stupid Iron to warm up? 😆