Monday, October 13, 2014

Rocket finishing - where most of the sanding happens

Here are 3 of the laminated fins for the Rogue before attachment. This picture is important, not so much due to the fins, but because of the makeshift tool just above them. Consisting of a strip of 400 grit sandpaper glued to a popsicle stick, it is very useful for sanding those small or hard-to-get-at areas of the rocket – in this case, the edges of the Rogue fins. You will often find yourself constructing such tools as you move on to harder, more elaborate builds or HPR; for example, I know quite a few rocketeers who glue sandpaper strips to dowels to help them sand smooth epoxy filets.
The Rogue fins and launch lug were attached to the rocket and filleted, just like the Aero Hi. After the glue dried, I applied sanding sealer to the balsa parts of both rockets to fill the balsa grain. There are a lot of different ways to do this, but no matter which mehod you choose, it will involve multiple coats and a fair amount of sanding. My technique is as follows:
  1. Go outside on the patio, or open the sliding glass doors to the work room. Aerogloss sanding sealer produces a lot of fumes, so you want to work in a well-ventilated area.
  2. Using the appropriate size brush (big for big fins, small for conduits, etc), apply a coat of sanding sealer. Wait 15 minutes then apply a second coat.
  3. Wait 45 minutes, sand with 320 grit sandpaper.
  4. Repeat step 2.
  5. Wait 45 minutes, sand with 400 grit sandpaper.
If you have sanded the sides of your fins before cutting or removing them from the balsa sheet, you should have a nice, glassy-smooth surface.  If not, apply another coat of sanding sealer, wait 45 minutes, and sand with 400 grit. Repeat until the grain is filled and the surface is smooth. Unless the balsa grain is deep, I usually find 4 coats is enough for fins; nose cones usually take 5.
After the sealer has dried overnight, you can then prime the rocket. Choice of primer varies from rocketeer to rocketeer – I use Krylon Industrial, again because it is forgiving, but others swear by Rustoleum, Valspar, or even Walmart brand (cheap, but it works). Some even try fill the grain and prime in one step by using Kilz sealer-primer. I tried this stuff once, when I built my Thrustline D-Region Tomahawk – it turned out to be Chuck Pierce’s worst nightmare, taking me 2 days to sand out all the bubbles to get a smooth finish. Plus, you could have probably buried me in all the dust that was produced.
Anyway, apply a coat of primer to the rocket, wait a few hours or a day or so (depending on drying time), and sand most of it off with 240/320 grit sandpaper. The pic below shows the bottom part of Rogue after this has been done.
The primer not only helps prepare the rocket for the paint, it also shows any imperfections in the finish – nicks, especially deep grain, scratches, uneven fillets, and so forth – enabling you to take corrective steps. For uneven fillets, I usually add another layer of glue, smoothing it out with a damp finger. Balsa nicks and scratches, as well as many other things, are usually fixed by applying thinned Fill N Finish to the area. This Elmer’s product is indispensable in producing good looking mod rocs, so be sure you go out and buy some. In the pic above, you can see where I have applied Fill N Finish to the non-papered edges of the Rogue fins to cover the cracks and imperfections. I am also building the BMS Mini Bomarc, which is shown below. Note that I have used Fill N Finish to fix nicks and scratches in the balsa wings, as well as an edge defect at the bottom of the right wing.
After fixing the imperfections (be sure to sand the Fill N Finish smooth), apply a second coat of primer, let dry, and sand with 400 grit. The rocket should now feel as smooth as a baby’s bottom; if not, you have more priming and sanding to do (more the latter I’m afraid).

1 comment:

  1. Yeah, Kiltz is bad stuff. It cloggs up the sandpaper! Chuck Pierce hates sanding and sealing and sanding. Laminating balsa fins with paper is an easy to avoid sanding.