That's the only stuff I use--it's the only stuff I can find at my local Ace.
I mix it with 3 parts CWF and 1 part *cold* water, and mix, and then maybe add a tiny bit of water as needed. When I used hot water, the CWF would just dissipate...
That mix goes on a little thick, but the alternative was to have it more watery and take more coats...I place the fins between wax paper, but paper towels on top and then put the fins between heavy books, to ensure they dry flat.
If you've never tried it, here's a technique you might consider... it works wonderfully for me and many others here on the forums...
Take a dab (bout a teaspoonful to a tablespoon) of filler from the can, and put it into a small "pop top" type container (I keep those little red-lidded snap-top reusable cups that the side items come in at KFC, but any similar small "resealable" container will work just as well). Get a 1 inch wide fairly-stiff bristle paint brush (just a cheapy off-the-shelf paint brush from any hardware store will work fine, don't really want an "artist" type brush) and then add water a couple drops at a time, gently working it into the filler using the paint brush by "stabbing" at the lump of filler to work the water in... keep working the water in a little at a time, until you end up with filler that becomes a smooth and even paste, about the same consistency as bottled hot dog mustard or pancake batter... IOW, a THICK liquid, not so thick that it won't collapse under its own weight back into the bowl if pulled up into a "peak", but not so thin that it runs like water either...
Now, you can BRUSH this mixture onto the parts. I usually go over them twice, to make sure everything is coated evenly and with enough thickness that it ensures everything is covered well. You'll sand 95% of this off subsequently, so no need in going TOO thick, it just makes more work for you later if you do. Just make sure everything is evenly and uniformly covered. Not really that hard at all.
I usually paper fins, because its faster, easier, and adds a LOT more strength, BUT, if you want to use filler on the fins, is you do BOTH sides at the same time, there's really little/no danger of warpage. At least *I* have never had any warpage because of it. The moisture you added really isn't as much as one would think, and it rather quickly evaporates and the filler tends to pull the moisture up and wick it to the surface exposed to the air, rather than into the wood, so I've never seen the need for complicated "weighting/pressing" methods myself...
If you're working with especially large/thin fins and worried about possible warpage (which is admittedly more of a problem with larger and thinner pieces of wood) and one wants to add a bit of extra strength, one could harden the balsa fins with CA before applying the filler... I use "ultra-thin" pink bottle CA from Hobby Lobby, rubbed onto the surface with the tip of the bottle applicator. If you plan to glue the fins on using wood glue, be sure you do a double-glue joint first application of yellow wood glue to the fin root edge FIRST so that the CA does not seal off the wood grain and cause poor adhesion and bond strength of the yellow glue later on. Apply a thin layer of wood glue to the root edge of the fin and allow it to dry-- it will draw itself into the wood grain and cure, and the subsequent CA application will not affect it. Then use a standard double-glue joint to attach the fin to the rocket.
Works like a champ...
Good luck! OL JR