FAQ5: How do I waterproof my plane?
You don't want water inside the plane for several reasons.  

• Water is heavy.  A slug of water sloshing around can make an airplane fly very strangely, or it may make it completely uncontrollable.  

• Water supports microbial growth.  Leave a soaked airplane  in a warm garage for a week and you'll have a science project that used to fly.  
• Water is corrosive. You really don't want the electronics getting wet.  

How do I keep water out?  
1. Seal all the obvious openings.  Make sure the covering is well-secured.  Scotch-tape is pretty effective when necessary. Tape hatch-covers, for example. 
2. Use an internal or at least a covered charging jack.
3. Mount your switch internally, and actuate it with a pushrod.  Usually pushrod exits are not too much of a problem, but nyrods do very well at excluding water.  Or small tubular guides work similarly well for wire pushrods such as the throttle pushrod.  Small tubular guides also work well for excluding water where pull-pull connections exit the plane.  
4. Use silicone seal around the holes in the firewall where the fuel lines pass through.
Seal the wing saddle with silicone.  Wrap the wing with plastic food wrap.  Lay a bead of silicone on the wing saddle and install the wing as if ready to fly.  Wait 2-3 days, because the silicone cures slowly between monokote and saran wrap.  Remove the wing, peel off the saran wrap, and you'll have a custom-formed gasket on the wing-saddle.  

What about when the water gets in?  
It will.  

1. Seal the wood inside the plane.  Water-Based Polyurethane varnish works well.  
2. Mount all the electronic components off the floor.  If some water gets in and starts sloshing around, it can just flow past the electronics without being dashed into them. Don't mount the electronics against ANY interior surface...you do loops and rolls, and the water might be anywhere.  
3. CorrosionX is a dandy corrosion inhibitor that will not hurt the electronics.  Available at most marinas.  Treat at least all your connectors, but some guys flood their receivers and servos with the stuff.  
4. Wrap the receiver in a baggie, rubber glove, or condom and tie something around the wire bundle.  No silicone here.  The acid in the silicone is corrosive to the electronics.  If the plane takes a dunking, remove the bag and be sure no water got inside.  
5. At the center of gravity, on the bottom of the fuselage, put in several folded paper towels.  Any water that gets in will be absorbed at the CG where it won't hurt the balance.  Yank the paper towels after flying and the water leaves with them.  
   

What about the floats?
Use Foam-core floats.  They cannot fill up with water.  They are light, strong, and durable.  Typically these are sheeted with balsa and ply, sealed with Water-Based Poly (Solvent-Based Poly eats the foam) or epoxy/fiberglass.  Then they are covered with  one of the heat-shrink coatings, or painted.  There are some good molded fiberglass floats, but if you use those, provide for a drain hole.  Plywood floats are strong but heavy.  See the science-project remark above.   Blow-molded plastic floats are the last choice due to their poor rigidity, poor shape details, and reputation for leaking.