OK, so that raises the question: what *is* "sanding sealer" and how does it relate to "dope"? Are these the same thing? Variants of some general kind of substance -- that is, is "sanding sealer" a *kind* of dope?
When I read about CWF & dope, I always see the same summaries: CWF is much more pleasant to deal with than dope. But then I see little hints that CWF might not be *quite* as good, in terms of providing a glass-smooth surface, as dope. Water-based CWF can warp a thin balsa sheet (such as a fin). It's not waterproof. You might need more coats.
Given all this, I wonder: why deal with CWF? I used dope in the 70s. I didn't use much -- it doesn't take a lot to coat the fins and nose cone of a small model rocket. The solvent evaporated into fumes, but I did not think it was a big deal, as long as I worked in a ventilated space. If CWF is almost-as-good as dope, then, heck, use dope. CWF "fixes" problems with dope that I don't find to be problems.
Or am I missing something here? (That is, a reply of the form, "Do whatever works for you," is not a useful reply. The point of this thread is to explore the merits and tradeoffs of different sealants.)
I also hear weight comments -- some sealant is lighter or some other sealant is too heavy. I'd really like to see some *data* on this. Has anyone used a *scale* to weigh how much these sealants add to the weight of a rocket? It would be really fantastic to know what the weight cost of a range of sealants is, in g/m^2. Then you could take the surface area of your fins or nose cone and work out what these sealants cost, in terms of mass footprint. I'm going to be a little skeptical of weight claims, pro or con, until I have some actual numbers. After all, if you do it right, you are going to wind up with the thinnest layer of sealant imaginable.
Finally (and this is relevant to my weight comments above), why not just seal with one of the laminating epoxy resins used to lay up fiberglass? You could put on a really thin layer because these resins are so liquid. You'd get a very hard, smooth, tough surface. Epoxy is relatively heavy, in this space of possible sealants, but, again, you are going to sand it all away, leaving only a tiny amount.
Everything I've said above is conjecture. I'm just getting back into messing with model rockets after a forty-year absence. One thing I'd like to do is check out some of these possibilities, with measurements, to pin down some answers.
But if anyone has done it already, that is much better.
EKH
OK, I'll toss in my $0.02, mainly because nobody really addressed the epoxy issue that you raised. But first...
Weight of finish is important, but it is not the overriding factor. Possibly the most important issue is to minimize ugliness drag, and ugly is in the eye of the beholder. There are many factors to "explore the merits of". Just a few are: weight, cost, time, availability, smell, ventilation, durability, coloration, availability, water proofing, drag reduction, wet sanding and other personal preferences. So yes, whatever works for you, is probably the right answer. I guess I'm an old doper from the late 60's, so I guess that sets some of my biases. I actually started in Control Line airplanes before model rocketry. To me the ugliest finish is when someone brushes on coat after coat of sanding sealer, until the balsa grain is undetectable, and never sands. Don't be that guy.
Cost is not a huge issue for me, but honestly , nobody wants to read the instructions in that new $5 starter kit that says to assemble and and finish this rocket properly you will first need to buy $100 of supplies. On the other hand, If you have lots of prime automotive finishing products at hand, why not use that? I used to be a serious contest flyer and I would typically have to design ,build, and finish say 10 models in 10 days, while working full time. Classic dope finishes work well in that scenario. I rarely have time to spray on some sandable filler/primer and let it dry a couple days between sanding, although I did that once on a scale model.
The most time consuming and weight critical models are boost gliders, rocket gliders,and helicopter models. PD,SD, altitude, etc are quick builds, but still have to be done; weight is not as critical and you can often get by with plywood fins. The lightest way to add color to balsa for visibility is to use a dye, next best is ink, then colored dope, then colored acrylic, enamel, lacquer, and finally epoxy. I like to cover all but the lightest of my models with colored Jap tissue. This adds strength and stiffness, but it may just be one of those personal preferences. You can also use something like Monocoat, but that gets rather heavy for typical contest builds. The dye and ink should still have a thin coat of dope or sanding sealer to smooth the surface and offer some protection against water and moisture. I reject general finishing with CA under personal preference, so I have no idea where it fits in mass wise. It would be good to have all of the viable finishing options quantified with grams/square meter values, but such numbers would vary considerably by the individual doing the quantifying, and even by different samples from the same person. Mark Bundick wrote an excellent article for The Leading Edge Newsletter about dope finishes. I don't remember the title or issue, but they are available on line, and perhaps someone can provide the link. The thesis is that dope is evil and how to use as little of it as possible.
The lightest glider that I have built many of is the Bumble Bee with a 1/16 inch thick wing. I'll cut out the wing and sand the lower surface smooth, then I'll paint 3/4 of an inch of the training edge on the lower surface with clear dope or sanding sealer. Let it dry and sand off the fuzz. This is to help keep the paper thin trailing edge together. Without this step the trailing edge will eventually separate along the deep grain, looking like a separate feather. After sanding the airfoil, the trailing edge will warp down just a little, increasing duration performance. About a 1/4 inch of the leading edge gets one coat of dope or sanding sealer to help toughen it against gliding impacts. This might be a place where CA could be used instead of dope. On larger, heavier gliders I may add a leading edge of hard balsa, bamboo, basswood, or even spruce. I'll finish the Bumble Bee wing with some magic marker coloring, and then give it a final smoothing with 600 grit sandpaper.
Something like a Delta Katt typically gets two coats of clear dope to seal and smooth the surface, but no attempt to fill the grain. A Jiskra (evolved) gets the wing tissue covered and the tail sealed with 2 light coats of dope. A Groundhog, depending on size may get one layer of tissue on the inner 2/3 of the wing, and a second layer on the inner 1/3 of the wing. The stab gets tissued if I add a pop up DT. I could go on, but my point is that the finish is intimately tied to specific models and requirements, not one solution fits all.
to be continued
) I used dope on my fins. Dope-sand-dope-sand-dope-sand-paint. I got very nice results. But I've switched to a light weight filler that also gives me very nice results and with less work, fill-sand-paint. My filler is porous, so I might do better with fill-sand-dope-sand-paint.
