Preventing warp to glued on fins?

Please excuse my silly fin question. I've been so focused on HPR G10 TTW to PVC stuff, that I'm finding surface mount the balsa to cardboard thing very challenging.

I managed to glue the fins on my Estes Patriot (2056) before sealing them with FnF...

The fins are not exactly perfect in alignment, but being the AR meathead that I am, I would hate to seal the fins the wrong way and cause even more tweakage to my 1st attempt at a hobbyist point of view.

What would be the best mixture "analogy" of a water to "Fill n Finish" to seal balsa fins without warping them while mounted on a BT? Mustard, Mayo or just plain chunky undiluted FnF?

Two comments. 1: Seal first before assembly. Then touch up the joints with FnF fillets over the glue fillets.

2) Too late for that now. Here is what I would do. Paint FnF ketchup on one fin only. (Actually 1 side at a time waiting until it is mostly dry before doing the other side.) Then, when the fin is dry but you can still sense dampness in the balsa, place the fin against a flat elevated surface and cover it with another flat surface and some heavy weights. Place some little objects under the tube, etc., to ensure that there is no twisting tension. Let this dry overnight. Repeat for each fin in turn.

I've actually done this, but dude: seal first! I have no idea why all kits have instructions saying to seal at the end. You can keep the glue joints naked (actually, just sand them naked before gluing).

Geof

Im with Geof!! Every model I build I seal the fins BEFORE I put them on. I thinks its even better when the suppliers give you a sheet of the balsa to cut out yourself. This was, you are sanding a solid sheet of balsa, not sanding on a fin thats been cut out and mounted, much more difficult to sand this way. Sand the edge to be mounted, glue to the body,and then seal the leading edge once the glue is dry. I like polyurethane for sealing my balsa, makes the fins like plastic, and gives it a glass finish once you throw paint on it. Heck, sometimes I think the wood looks so good with the natural finish, I dont even paint the rocket!!!

WOW,I cant believe I have been doing it wrong for 20 years!!! I never seal the fins untill im done building,and have never warped fins after they have been on. I also use finishing epoxy as sealer for my plywood fins.

A lot of warpage happens from doing one side, letting it dry, then doing the other side. Sealer and dope and paint tends to cause shrinkage as they dry. And uneven shrinkage causes warps.

Or, applying more sealer or dope or paint more on one side than another.

Since you have the fins glued on, I’d suggest applying the sealer to one side of a fin, make sure the sealer will not run or drip, then apply sealer to the other side of the same fin. I would not let one side dry, and then apply sealer to the other side, that is how warps can happen.

I agree about doing the fins first, then glue them on later. I love to sand the fins using a flat sanding block, it is almost impossible to sand them well if the fins are attached to anything first. That would be almost s bad as sanding a shape into the fins (rounding the edges, or worse yet trying to airfoil them) with the fins attached first, rather than sanding the shapes with the fins separate, then gluing on the fins.

For applying sealer or dope, I like to apply to one side, let dry just a bit until “damp”, then apply to the other side. And when done, place the fin onto a couple of spacers so when I lay be fin flat on a table the fin is not really sitting totally flat against the tabletop, the spacers hold it up so some air can get up underneath so the underside can dry too. A couple of square cross section toothpicks could do, but I usually use some balsa scrap cut into narrow strips. After awhile I will turn them over too, so the underside is now face-up to get better exposure to the air.

And let the fins air-dry. Do not try to fast cure them with any type of heat.

When you are done sealing the fins, and have done your last coat of fillercoat or whatever you use for the last finish coat, and are happy with how smooth the fins are.... let them sit for another day or two to let the sealer or dope dry even more. At least if you can stand the time to wait. So that after you do later glue the fins onto the tube, and paint the rocket, there will be more time that will have elapsed before the paint goes on. This is so that there will be less chance of the fins warping due to the sealer and filler finish inside NOT being dried enough before being painted.

On some models, I like to glue the fins on with CA, then go back and use Titebond to create fillets. When I do that, I will do a Titebond fillet on both sides of one fin, and lay the rocket horizontally with that fin sicking straight up, so the fillets will not run or drip. After letting air-dry for 30 minutes or so, the Titebond is set up enough to be able to apply fillets to another fin root, then lay the rocket on its side for another 30 minutes, and repeat. Sometimes I do more than one Titebond fillet layer per fin.

Or course, on a lot of other models I use thick CA for fillets, or 5 minute epoxy. It depends on the models and the situation. Often the priority is more to get a model built and flyable, looking “OK”, than to worry a lot over getting a great finish on it. Whenever I finally get around to building one of the Interceptor kits I got last year, I will glue the fins and wings on with CA like I do with most models. But for the fillets, I definitely will use Titebond and not thick CA or epoxy. And I will allow plenty of time for things to dry before painting.

Anyway, I mentioned the Titebond since if anyone else also uses an air dry type of glue, which will shrink as it dries, you have to allow several days for the glue to dry so much that it won’t shrink anymore after you paint it. Because if it is not dry and shrinks later, it will look funky, and the paint over the fillet may crack.

Of course if I had to build a rocket tonight, to fly tomorrow.... I would not bother with all of that.

- George Gassaway

I always do my FNF while the fins are still on the flat balsa sheet and do both sides at once. That allows the moisture to basically saturate the fin from both sides and it's sort of self regulating.

If you do one side then let it dry then possibly the FNF sunk in more than half the fin thickness. Next time you coat the other side then it won't pull it back straight. At least that's my experience.

..Pat Butler said:

I always do my FNF while the fins are still on the flat balsa sheet and do both sides at once. That allows the moisture to basically saturate the fin from both sides and it's sort of self regulating.

If you do one side then let it dry then possibly the FNF sunk in more than half the fin thickness. Next time you coat the other side then it won't pull it back straight. At least that's my experience.

Click to expand...

I feel that the balsa can end up warped if you do both sides at once and let it dry. Heck, the balsa can be warped straight out of the bag. I used to do it the way suggested by several people here: paint both sides and airdry. Then, I decided that I preferred pressing. You can't press when the FnF is wet, or it mars the surface. So I would paint both sides, wait until dry on the surface but damp in the wood, then press. Finally, I realized that it would yield the same result if I painted one side, flipped when barely damp, painted the other side, then pressed when dry on surface but damp in wood. The only reason to prefer this over the both-sides-at-once-and-press approach is that you don't have to balance balsa pieces on pointy corners to dry. You can just lay them flat. And you can find places to hold the piece while painting. With the fins already on the rocket, he could easily paint both sides at once, wait until barely damp, and press.

My main point was pressing. I think this is by far the best way to get perfect flatness. I have even taken sealed fins, sprayed them with water, and pressed them.

BTW, I sort of agree that doing the FnF on the whole sheet is good. It sure helps with sanding. But...what do you do about beveling? On several recent projects I filled the sheet very carefully, then cut out the fins/wings, then sanded wide, gentle bevels. I realized that I had sanded through the filler down to wood over maybe 30% of the surface. My original work was somewhat wasted.

BTW

Call me stoopid I guess,but all this seem like so much overkill,I have never warped balsa or plywood. I dont even know what FNF is,yes fill n finish or something like that. I seal my fins after gluing to rocket. I know everyone has their own way,but I just cant see putting that much time into a rocket. These are just my own personal reflections,and YMMV.

Jim:
I'm with you, I can't recall the last time I warpped a balsa fin or wing. prehaps is because I don't use FNF either.
but do remember some models can take a very long time to complete

I do agree with finishing both sides of balsa at the same time, If papering or tissuing balsa, It's important to get material on both side while both sides are moist.
If using balsa filler or sanding sealer It's usually done on the model, applied to both surfaces.
When doing large wings for gliders or rotors of Helis, I do like to fill, coat or primer them before assembly but it's not that big a deal.

The trick I believe; Regardless of mounting method or material used (before or after assembly) is always coat both sides at the same time.

I like to use paint. I sand the first coat down a bit and then hit it again.

I once spent several hours sanding down three fins on an Estes Star Dart clone. Nicely airfoiled and everything. Rocket was painted, almost museum quality (except for the that one run), loaded a C6-7 in her; hit the button and never saw those fins again.

Lately I just am okay with just rounding off the edges to help the paint adhere. I rarely even sand the fins smooth before painting. I still lose a few from time to time though. Last one just vaporized. Every other rocket landed within 75 yards of the launch pad. This one flew high, saw the ejection chalk and then nothing.

Joe

Hey, wow! Thanks for all the ideas and advice folks!

Methinks I'll just baste up my Patriot with already mounted fins and go from there.

I did manage to pre-FnF the fins for my 220 Swift last night. Turned out pretty good but still a little grainy in appearance. A few coats of primer oughta clean it up enough to be tolerable.

I've got a small pile of balsa cut-outs going, I think it's time for a little experimentaion. I wonder how well acrylic (Future) would work in lieu of Polyurethane? We'll see...