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CA on Balsa
#1
I know CA soaking is done to harden balsa fins but does it also fill? Should I CA the fins then use wood filler? or just one or the other?
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#2
CA will "seal off" the pores of the wood in the sense that other materials cannot penetrate them, ie wood glue, which gains most of its bond strength by penetrating into the wood pores and bonding with it as well as mechanically bonding to the other material on the other side of the joint. Hence why it is NOT good practice to "CA" balsa BEFORE gluing the fins onto the rocket, or using CA to 'tack' the fins onto the rocket and then using an external application of wood glue to "complete" the joint...

CA (in its ultra-thin form) will wick deeply into the large grain and pore spaces of balsa wood, and cure when exposed to the slight remaining amount of moisture present there... Once it "sets up", it closes off the pore spaces and "seals" them. Note that thick or gel CA cannot wick deeply into the wood due to the high viscosity... therefore they should only be considered as a "surface application" and little/no hardening of the underlying wood should be expected to occur...

Now, CA is some funny stuff... it will tend to raise any uncut wood grain "hairs" or remaining balsa dust (of which there is ALWAYS some remaining balsa dust present, no matter HOW carefully a balsa part is cleaned or prepared, or how smooth it is... I suppose it hides in the entrances to the wood "pores"), and then harden them in this raised condition... thus, when one "seals" balsa wood with CA, it will tend to create a ROUGH surface texture as the CA hardens these surface imperfections and magnifies them. At any rate, the CA itself seems to bubble up slightly and harden into a rough 'crust' on the surface of the wood that's been treated with CA wicked into it...

The best procedure is to plan on sanding the part thoroughly AFTER the CA has been wicked into the wood and its had time for the CA to fully cure... Usually doesn't take long... personally, I sand with 220 grit sandpaper followed up with 440 grit... this effectively smooths out the rough surface and prepares it for a final treatment with Elmer's Carpenter's Wood Filler (CWF) thinned with a little water to the consistency of hot dog mustard and brushed on the surface with a paint brush and allowed to dry. An alternative is to use Bondo Glazing and Spot Putty (BGSP), which is basically red oxide primer solids thinned with a little bit of lacquer thinner until it's about the consistency of toothpaste... personally I prefer the CWF for this step...

Allow the CWF to dry from a couple hours to overnight (depending on your local conditions at the time of application) and then sand down with 220 grit followed by 440 grit sandpaper... inspect thoroughly and then follow up and repair any remaining imperfections with either another application of CWF, or BGSP, or if the surface is smooth and satisfactory (no large blemishes-- doesn't have to be "perfect" as primer will be the next step) go ahead and give it 2-3 coats of a good spray primer, allow to dry, and then sand with 220 grit followed by 440 grit again... as a final step on CA sealed balsa nosecones and transitions, I "damp sand" the primer using 440 grit sandpaper dipped into a bowl of water and shaken off to remove MOST of the water... the remaining moisture will combine with the liberated particles sanded off the primer and form a "sanding mud" which results in a MUCH smoother surface than can be obtained with dry sandpaper alone... this can then be subsequently wiped off with a damp paper towel, and the part dried with a dry paper towel...

Using these methods, one can make even the coarsest looking balsa cone or transition look as smooth or smoother than plastic...

For fins, I generally prefer to "paper" them by gluing a sheet of printer paper to the fins after they've been cut out and stack sanded to identical size, and any aerofoiling sanded into the leading and trailing edges. This is accomplished by cutting up a sheet of typing paper into pieces more than double the size of the fin, applying a THIN (the thinner, the better, but even application is important-- spread the glue thinly and EVENLY with your finger) application of WHITE GLUE (yellow glue shrinks too much and dries too fast) to the paper, sticking the fin down onto it with the leading edge toward the center of the paper, then applying another THIN, EVEN COAT of WHITE GLUE to the other side of the paper or the fin itself, and then FOLDING IT OVER onto the other half of the paper OVER THE LEADING EDGE... then burnish the entire paper down TIGHTLY against the underlying wood fin starting from the leading edge and working toward the trailing edge, and from the center working toward the root and tip edges... this will ensure that the glue is evenly distributed and the paper is sealed down TIGHT against the wood, and the remaining glue is squeezed out around the edges... Allow the fins to dry overnight, then carefully cut away the paper extending beyond the edges of the wood with a SHARP hobby knife, and lightly sand the edges of the fins to "shave" the paper down flush.

To glue the fins on the rocket, I prefer the DOUBLE-GLUE JOINT method using wood glue. To do this, apply a THIN, even coat of glue to the root edge of the fins, and set them aside... (I prefer to use an old egg carton flipped upside down, with slits cut across the egg cups to make "holders" for this step) and then also apply a thin, even coat of glue to the fin lines on the body tube where the fin root edges will eventually be glued. Note these are THIN applications-- in no way should these be thick, runny applications of glue! Once the glue has cured for 20-30 minutes, another THIN application of wood glue is made to the fin root edge, over the last glue application... the fin is then IMMEDIATELY lined up with the fin line, and touched down onto the tube... it will virtually instantly "lock" down to the tube, so be careful and make sure your alignment is correct before touching the fin to the tube... Then the other fins can be similarly attached. Usually within the time it takes to apply the other fins, the glue joint is strong enough to go ahead and fillet the fins; I prefer to use Titebond Moulding and Trim Wood Glue for this (TMTG), as it's a no-run, no-drip formula of "thickened" wood or white glue that will not appreciably shrink and is perfectly shapeable and conformable to the rocket fin fillet and launch lug fillet when applied, and ALL the fillets can be done at once, and the rocket set aside to dry. No sanding or other work on the fillets is required, because you can easily shape the fillet to the desired size/shape with a damp fingertip as you apply them. Note, that if one DOES desire a thicker, heavier, rounder fillet, apply it several steps-- apply thin, fillets and once dry, go back over them with a second and third application if necessary to build up the final desired fillet thickness... since they can all be done at once with short dry periods between applications, this takes a lot less time than one would expect...

Later! OL JR Smile
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#3
Excellent   reply! thanks! Lots to absorb....
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#4
   
Here's some pics of how I finish cones and transitions...

First, the materials and setup... 3 cones (from a Dr. Zooch Crumman 3C kit), a cardboard box lid with wax paper insert (to prevent sticking of the cones during curing) and some ultra-thin pink bottle CA from Hobby Lobby (available with the 40% off coupon-- a real money saver!)
   

Next, applying the CA... holding the cone by the shoulder, tip down, I gently wet the surface of the cone with CA and gently rub it a bit with the applicator tip to spread the liquid evenly across the surface... continue until the wood is saturated, gently turning the cone as you apply, keeping it tip down so any that runs off will not glue your fingers to the cone... be sure to keep moving your fingers to prevent them from being glued to the cone, as some CA CAN and WILL wick up to the base of the cone and can glue your fingers down to it if you don't keep moving and lifting your fingertips off the shoulder edge one by one as you work! 
   
   
   
   

Once the wood has absorbed all it will take, set the cones aside to dry... setting them on wax paper is recommended so they don't stick (as much).  Warmth and moisture will cure CA rather quickly. 
   

Once the cones are thoroughly cured, they can be sanded with 220 grit followed by 440 grit... this will remove the "hardened balsa hairs and dust-grit" that makes a rough surface on the cones...  This can be followed up with a treatment of thinned Carpenters Wood Filler (CWF) thinned down with a little water worked into it with a brush so it's the consistency of hot dog mustard... In this case, I was experimenting with the Elmer's Stainable Wood Filler-- which I DO NOT recommend-- it has 'microballoons' in it that stick out and sand like lava rock bits glued to the surface of your cone... use the regular "Elmer's Carpenter's Wood Filler"... it works MUCH better but I don't have a picture of it for some reason... These cones are from a Dr. Zooch Space Shuttle kit...
   

Once this has dried, it is sanded off with a 220 grit followed by 440 grit sandpapers... so long as all the major imperfections are filled to satisfaction, the cones are now ready for primer... I start by taping off the cone shoulders with masking tape, and tape the cones down to "paint sticks" made from either paint mixing sticks from the lumberyard, or just strips of cardboard as seen here... a couple bits of masking tape on either side will hold the cones securely for primer application... Generally I give them about 3-4 coats of primer, starting fairly lightly and getting heavier with each successive coat... I generally wait about 10-15 minutes or so between applications.  Then allow the cones to dry thoroughly, preferably overnight (but in good drying conditions, a few hours may be enough).  These cones are from a Dr. Zooch EFT-1 kit...
   

Next,  the primer I use... note that it's "fast drying" and WET SANDABLE...
   

Next, the cones are sanded down with 220 grit followed by 440 grit, dry... most of the primer is sanded off, and if necessary, more primer is applied to fill imperfections.  Usually a single primer/sanding treatment is enough if the previous prep work has been done satisfactorily... Notice how the dry sanded cone still has deep sanding scratches in this close up...
   

More to come!  OL JR Smile

It's important to sand properly... do not use too much pressure-- let the paper do the work... and also, if at all possible, ALWAYS sand in small overlapping circular motions, NEVER in straight lines... I like the continuously turn the piece I'm sanding in my hand (in the case of cones and transitions) while gently sanding in small circles with the other hand... NEVER sand in a single spot for any length of time if you can help it... ALWAYS be moving around the piece to keep the sanding smooth and even...  That way you'll minimize or eliminate sanding in flat spots and deep sanding scratches to the extent possible, and make it easier to remove them satisfactorily...

Next, we switch to "damp sanding".  First a bit of an explanation... guys who restore old cars or put killer paint jobs on newer ones use a technique called "wet sanding" to achieve those ultimate "mile deep wet look" finishes... we can achieve something very similar using basic tools and techniques slightly modified for average rocket materials and methods... True "wet sanding" usually involves long straight sanding boards or sanding sponges to adapt to the flat expanses and compound curves of automobile bodywork, and usually a dribbling garden hose for the water supply, dribbling into/onto the area being wet sanded and running off, taking the removed primer or paint particles with it... While this is fine for metal or fiberglass autobody work, for our generally cardboard and balsa rockets, this would be a recipe for disaster... SO, we adapt the technique... Use as little water as possible, and be very careful where and what is exposed to that moisture.  Hence, "damp sanding".  We don't need running water; just enough to moisten the paper is sufficient... you'll need an old towel, some paper towels, a small bowl of water, and your sandpaper, a 440 wet/dry paper is what I use...
   

The sandpaper is dunked into the bowl of water periodically, and the excess water is shaken off or daubed off onto the towel, so that the paper is just damp or moist to the touch, like a moist towelette you'd get at a wings joint or something... then sanding proceeds in small circles as the part is rotated in your other hand... The moisture from the paper will combine with the liberated primer particles sanded off to form a "sanding mud" that will accumulate on the surface.  The moisture lubricates the paper and prevents the particles from packing up the sandpaper, and act as a lubricant/polishing compound while you sand...
   

Eventually, though, the moisture will dry out and be absorbed by the primer particles, and the sanding mud will get thicker and thicker, and eventually the paper will start to clog... when this happens, dunk the paper in the water, and rub your thumb across the surface of the grit while the paper is submerged to loosen and free the trapped primer particles clogging the grit of the sandpaper... then dunk the paper a time or two again to wash off the remaining particles, daub off the excess water on the towel, and return to sanding... If the "sanding mud" gets too thick on the piece, wipe it off with a damp paper towel...
   

Once wiped off with a damp paper towel and dried with a dry paper towel, you can now see that the sanding scratches are virtually gone, and the surface is nearly silky smooth... you can repeat the process to the point the surface is as smooth as you desire...
   

Once the piece is sanded to your satisfaction and cleaned off, it's time for a final inspection... hold the cone, transition, or fin up in front of you at eye height, and sight over it toward a bright light source such as a lamp, overhead fixture, or brightly lit window... Now, observe the "glint" of light off the surface as you gently turn it or tilt it to reflect light off the surface from the source to your eye... this reflected "glint" will show imperfections you cannot otherwise see or feel with your fingertips... much like how a CD reflects laser light back to a sensor, with the information encoded on the "imperfections" that scatter the laser light back in specific patterns... if the reflected "glint" is smooth and unbroken, you have a smooth, silky surface ready to take paint... if the reflection is "wavy" or has visible dimples or high spots (visible as shadows or bright spots) then you might want to work on it a bit more with the damp sandpaper, clean it off, and inspect it again... Any imperfections you can see in normal light looking at the piece WILL show through in the final paint job... Paint DOES NOT COVER imperfections-- it MAGNIFIES THEM!!!  So make sure that the surface you see in the final sanded primer is the finish you want on the final paint job! 
   
   

Once painted, the cones will look like plastic... Here's the final result, on the Dr. Zooch EFT-1 kit...
   
   
   
   
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#5
Verry Nice! I just busted out an old kit (Thrustline A-Slam) it has a large balsa nose cone. I think I use your method above....

Picture is not my A-slam....but what I want it to look like


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