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Old 03-07-2011, 04:21 PM
captain26 captain26 is offline
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Default Balsa Finishing?

BAR with balsa fin/cone finishing question? As a kid, I didn't worry much with filling the balsa grain in fins and nose cones and if I did, not much. Build it, paint it and launch it! Next!

As a model builder again, I would like to finish my models to a higher standard and have gained good info from this forum. I am currently using the Elmer Fill/Finish product and have had good results, not great? It seems that no matter how many coats I apply, some grain remains - especially on a nose cone I'm currently working on. Does the balsa need to be sealed before the filling process starts? Seems like the balsa is soaking it up like a sponge? Thanks in advance for any advice/info!
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Old 03-07-2011, 04:46 PM
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Yes, at some point you will need to seal the wood. Aerogloss Sanding Sealer or Balsa Filler in three coats with 400 grit between. Then a prime (I commonly use matte white to add good undercoat) then the main color probably in two coats. If you want to really have a slick fininsh then you would use a relatively heavy final coat for color depth. Then you will get into the clear coat converation.
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Old 03-07-2011, 04:51 PM
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For fins, I'm in the "self-adhesive label paper" camp. Stick it on, trim, and hit the outer edges with thin CA to seal them down.

For NC's and transitions, Kilz primer is a good method. You will sand much of it off, but what remains becomes a really good "skin" that can be polished with progressively finer sanding papers before the color shot is applied.

For final finishing/decal seal coating I use "FFF", or what used to be called "Future Floor Finish". I think it's been renamed to Pledge, but it's still the same stuff. The technique has been around since the stuff first came out. I airbrush it straight, no thinning. Works both as a decal surface prep, and as a decal sealer.

FnF is still a good surfacer, if just a bit messier. It's water-based, so you have to be careful on thin fins to do both sides evenly, otherwise the fin will cup with the grain as one side expands and the other doesn't.
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Old 03-07-2011, 05:28 PM
stefanj stefanj is offline
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My preferred method:

A sealer like dope Sanding Sealer or Minwax Wood Hardener

(sand smooth)

Wait. Wait longer. Let that stuff outgas.

Elmer's filler, several coats, sanding in between.

Wait a bit. Let that water evaporate.

Prime, sand, prime, sand until it is smooooooth. If necessary, use thick gray enamel to fill cracks and dings.

Wait.

Several coats of paint. Wet sand out any problems.

Dry thoroughly.

Wet sand if necessary. Rub out any haze or imperfections with polishing compound.

Future.

Decals.

More Future.

Buff.
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Old 03-07-2011, 06:32 PM
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Personally, I use Aerogloss Sanding Sealer exclusively. The number of coats depends on the hardness of the balsa and on the desired finish. This Texas Firefly took 4 coats on the nose cone and fins.
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Old 03-08-2011, 09:23 AM
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Here's my method that I have been using since 1977 (almost 35 years):

Phase 1)
3 coats of WELL-MIXED and STIRRED Aero Gloss Balsa Fillercoat with sanding between coats 2 and 3; coat two should be applied over coat one as soon as the solvents flash in coat one.
Sand coat two after an hour and apply coat 3.
After drying at least an hour, sand coat 3.

Phase 2)
After the sanded balsa fillercoat has dried overnight, apply two or more coats of Aero Gloss Sanding Sealer over the fillercoat, with sanding between coats after they dry.

Some apply the sealer before the fillercoat but that is BACKWARDS as the fillercoat FILLS much better over the raw wood.

Finished wood parts using this method can be topcoated with ANYTHING including butyrate dope. Using Elmers Water-based fill-n-finish results in a substrate NOT compatible with dope or "hot" solvent content lacquers. In addition, fill-n-finish makes a HIDEOUS DUST MESS when sanding which Aero Gloss products do not.
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Old 03-08-2011, 09:37 AM
captain26 captain26 is offline
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Great tips/ advice, thanks to everyone! I learned the hard way on my Nike X fins using the water based FnF. They are thin, fairly soft balsa and looked like Fritos when they dried! Glad I cut two sets, coated both sides at once on second set and they are fairly straight. As a couple folks noted, the FnF does make a dusty mess, had to move my sanding operation outside!

Is there a difference between Aero Gloss sealer and fillercoat or is it the same product?
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Old 03-08-2011, 09:54 AM
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Another method for sealing is using laminating epoxy. This is somewhat of an advanced technique, so I am only mentioning it as it is something that I do to seal balsa. If you add talc and/or glass microballoons to the epoxy, it helps to close up the tiny holes in the wood. Usually two treatments are all that is needed. This method as seen as overkill by some and it's use is certainly in the minority of builders. AFAIK, epoxy-based sealing methods will have no compatibility issues with either lacquer or enamel primers.

Also, on the topic of epoxy. Stay away from polyester and vinylester epoxies. There is some nasty stuff in them and those are the kind you typically find at home supply and auto supply stores. Aeropoxy and SYSTEM 3 by WEST are good quality epoxies, but there is a distinction made between laminating epoxies (which are quite runny for fiberglass or carbon fiber layup) and epoxy adhesives (which are thicker).

Regardless of whichever method you use to seal balsa, the final finish will be only as good as your primer coat. Good paint shows how good a job you did on sealing and priming.

Greg
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Old 03-08-2011, 10:34 AM
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Fillercoat is different than Sanding Sealer.
Balsa Fillercoat is high-solids dissolved in Clear butyrate dope.
Sanding Sealer is plasticizers and hardeners mixed with Clear Butyrate Dope.
Finish one gets with fillercoat is smooth but porous and soft.
Sanding sealer "seals" and toughens the wood leaving it with a plastic like top coat.
One can use either or, but they work best when used together as a system.
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Old 03-08-2011, 11:40 PM
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Quote:
Originally Posted by captain26
BAR with balsa fin/cone finishing question? As a kid, I didn't worry much with filling the balsa grain in fins and nose cones and if I did, not much. Build it, paint it and launch it! Next!

As a model builder again, I would like to finish my models to a higher standard and have gained good info from this forum. I am currently using the Elmer Fill/Finish product and have had good results, not great? It seems that no matter how many coats I apply, some grain remains - especially on a nose cone I'm currently working on. Does the balsa need to be sealed before the filling process starts? Seems like the balsa is soaking it up like a sponge? Thanks in advance for any advice/info!


Here's what works for me...

First thing when I get a kit, I inspect the balsa nose cone/transitions for smoothness... some are really nice, some tend to be pretty rough, like the balsa 'tore out' when it was being shaped, and leave a 'scalloped' surface behind. If necessary, sand with 220 grit to get it pretty smooth and even, but I'd say 80% of the time this step isn't necessary.

Then, using some ultra-thin CA (I prefer the cheap stuff in the pink-labeled bottle from Hobby Lobby, which you can get with the 40% coupon on the weekly special) wet the surface of the nosecone down with the CA (do this outdoors and stand upwind or crosswind-- the fumes can be rather eye-watering!) and I usually start at the shoulder end of the nosecone (only apply to the part of the cone OUTSIDE the rocket, not to the nosecone shoulder itself, as it will *slightly* increase the diameter and require sanding down to fit the tube well.) Apply a thin, even layer all the way around the cone, usually twice-- the balsa will "suck" the stuff right into the wood fibers. This greatly strengthens the wood and seals the pores as well. Carefully set the finished nosecone aside to 'dry' for about 10 minutes or so, on a cardboard box or wax paper (so if any glue runs down, it won't glue itself to what it's sitting on).

This tends to raise the dust and loose fibers on the surface of the nose cone, so a light sanding with 220 grit will get rid of all this stuff and make the nose cone very smooth again fairly quickly.

Take your Elmer's carpenter's wood filler, and using a small baby food jar or suitable container, mix about a teaspoon of it with several drops of water, and work it up until it's smooth and about the consistency of hot dog mustard. You want it thin enough to BRUSH on, but not so thin that it's watery and makes bubbles... just about like bottled mustard. Using a 1" cheapy paintbrush, apply the thinned FNF to the nosecone/transition and set it aside to dry.

Sand with 220 grit, taking MOST of the filler back off. Reapply if necessary, let dry, sand it down with 220 again. By this time the cone should be about 95% filled and smooth. Sand again with 400 grit to make it slick... you just want to get it nice and smooth and ready for primer-- if you can see any defects at this point, now's the time to fill them with another spot application if necessary.

Give the cone/transition a couple coats of good filler/primer (I like Walmart Colorplace Hi-Dep Red Oxide Primer, but they quit making it, so now you can only get the regular gray primer, but it works well too, and of course Dupli-Color and other "high solids" primers will give you and excellent job as well, for more $$$. The main thing is, make sure the primer is compatible with whatever paint you plan to spray over it.

Once the primer is dry, sand with 220 grit, until it's as smooth as you can get it. If you sand through the primer anywhere trying to get everything smooth, reapply, dry, and resand. Once it's smooth, switch to "damp sanding" with 600 grit wet/dry sandpaper dipped in a bowl of water and shaken off, use just enough moisture on the sandpaper to prevent it from clogging. Sand the whole surface in small circles, moving constantly and overlapping the previous passes. Periodically wipe off the 'sanding mud' (primer dust mixed with water) with a damp paper towel. Hold the cone up to the light and sight down it like a gun barrel, and slowly turn it... the 'glint' of light off the surface should be smooth, even, and unbroken, not wavy or showing any other visible defects. This whole primer sanding and resanding may take 1/2 an hour to an hour to do per part, but it's worth it.

Wipe the finished part down with a damp paper towel to remove all traces of dust, and wipe down with a dry paper towel and set it aside to dry overnight. They'll be ready for paint in the morning.

Once spray painted, the part should look identical to a plastic part. I've been amazed at how smooth balsa can turn out with a little time and elbow grease using these simple methods...

For fins, the same methods can work, but it's MUCH simpler to simple glue a layer of printer paper to the fins to cover the wood grain... and it strengthens the fins SUBSTANTIALLY where simple FNF does not. I cut and stack-sand my fins to identical shape, sand in any desired airfoiling, then get a few sheets of printer paper. Cut a sheet of printer paper into pieces about 3 times the size of the fin... enough to cover both sides of the fin at once and still have at least an inch of paper all the way around the edges of the fin. In fact I usually lay the fin on the paper, about an inch in from the edge, with the LEADING EDGE of the fin toward the center of the paper, trace it lightly onto the paper, and CAREFULLY ROLL THE FIN OVER THE LEADING EDGE, and trace it again to give a "mirror image" of the fin. Apply a thin, even coat of WHITE GLUE (VERY thin, but even!) to the entire surface of the paper where the first side of the fin will be glued down, extending past the outline of the fin, and gently press the fin down inside the first outline. Apply a second thin layer of white glue to the exposed surface of the fin (yellow glue isn't really any stronger here and white glue is MORE than adequate to the job, shrinks less, and smooths out easier, and is preferable on paper/wood joints instead of yellow glue). Once you have a THIN even coat on the entire exposed surface of the fin, gently roll it over THE LEADING EDGE onto the other half of the paper. Use a Sharpie marker or other 'fat' round pen or object (Sharpie markers are darn near perfect to the task, though) and gently "burnish" the paper down tight to the fin, starting from the center of the leading edge and working toward the root and tip edges, and back toward the trailing edge-- rolling/pressing the paper down tightly against the wood, and working any excess glue toward the edges of the fin. Flip the fin and repeat the burnishing, finally burnishing the paper down to itself around the edges of the fins, and set them aside to dry (usually overnight-- I do this the same day I do the nosecone filling/priming/sanding).

When dry, cut the fins out of the paper with scissors-- cut down to within about 1/4 inch of the edge of the fin. Using a SHARP hobby knife, gently 'shave' the remaining paper off the edges of the fin down to the balsa on the root, tip, and trailing edges (the paper should be tightly glued to the leading edge and smooth) and presto-- finished fins ready to glue on the rocket, suitable for priming/light sanding/painting. Having the paper wrapped and glued over the leading edge eliminates the possibility of delamination from the airstream ripping past the fins in powered flight, and the paper EASILY doubles the strength of the fin (or more) while adding only VERY MINIMAL weight... works like a champ!

When done right, the whole rocket will look like it's made of plastic... Also, tube spirals and any other imperfections that need a bit more than primer to fill, I find that the autobody 'spot putty' sold in tubes at auto supply stores works MARVELOUSLY on... just dab a bit onto the spot, and sand it down smooth when it's dry. The stuff is basically primer solids with just a bit of lacquer thinner in it to make it a paste...

Good luck! OL JR
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