Only problem is, stuff often won't stick as well as you would want and the stuff never stops shrinking. Can be a paint, though it might work OK with Monokote.

I've used this stuff with mixed feelings about its usefulness.  It is very soft and sands wonderfully.  In fact, I think it can be scraped off with a fingernail.  I still use it on occasions for very thin fillets and then only with a covering on glass and epoxy, and still, I've had bubbles form in the vicinity but unable to confirm that it is the Hobbylite.  Use with caution.

I'd give a shiny Penny to anyone who would post a fillet material which sands nearly as easily and strong enough to be safe.  I'm thinking of trying some experiments with a heavy dose of talc in a very thin epoxy.(Zap Finishing resin, one part A, one part B and three or four parts acetone).

Al

Why yes, Kim, I would be pleased to expound on the wonders of the blue stuff. This is a 2-part epoxy lightweight filler made for full-scale homebuilts, especially composite structures. The product name is SuperFil, made by Poly Fiber which I believe is a Stits division. It comes in two containers, one blue and one white. The two creamy parts are stirred together (in a separate container) to form a smooth paste that comes out light blue. It has a pot life of an hour officially but I find it starts to take a set at about 15 minutes in 60-degree weather. It tools easliy. I use sticks of 1/16 ply made with different radiuses to get the desired fillet cross section, then run a finger dipped in solvent (alcohol, acetone, thinner all work) along the fillet. Applied this way it requires hardly any sanding, but it does sand very easily for an epoxy product. I think the body of the material is mostly microballoons. Dries to sanding hardness in about 8 hours. This is a very superior product and the best fillet material I have tried. It is very light and does not shrink. It is made for a vibrating environment. And the icing on the cake? It costs $18.95 for about a quart, enough to build 100 models. And it can be shipped, because unlike dope it is not a "hazardous material." Got mine from Sacramento Sky Ranch in, you guessed it, Sacramento. Shipping adds about $8.

I am almost sure it is available from Aircraft Spruce also....

Spruce has it, dont be fooled though, when you pull the containers out of the shipping box, you will think they are empty, this stuff is pretty light!

That's where i ordered mine from yesterday.  "Aircraft Spruce".  $19.95 for a quart.

After trying several brands that never dried right or just did not have the strength
for a fillet .  I finally got smart and ordered the most talked about product being
used to build quality models.
It was made to be used on full size aircraft and is so light it can be used for
modeling.

The times I've used it, it dried up in the jar and had to add water to get it at a good consistency. I just got some Micro fill and it seems better so far as it is not dried up and apply's with a razer blade just fine.

 Lee,

  I bought the "Micro-Fill" and after getting it home read the whole set of directions. At the very bottom in the Red Band in Black Letters is a note. It states --- "When applying Butyrate Dope over Micro Fill, filler must be thoroughly dry and sealed with any polyester resin or epoxy finishing resin prior to applying Butyrate Dope" What are you sealing it with? Seems like an awful lot of extra work?

  "Billy G"   

Bryan,
For solid wood surfaces only, right?  No open bays.

Kim Mortimore

I guess we're dinosaurs, Pete.

I can''t remember which issue it was, but, on one of Bob Hunts Stunt News Videos there was an article on making filets using balsa wood.  He started with shet wood the thickness of the width of the filet.  Then cut it to fit the outline of the wing or stab that needed a filet.  Then used dowel rods with sandpaper wrapped around them for making the concave surface of the filet.  It looked like a lot of work, but, the end result was fabulous on his planes.  Later,  DOC Holliday