asianflava wrote:I used West 407 which I've read is microballoons based (whatever that means)
Microballons (my-crow-bah-lon, a French word) are one of the four main fillers that you might use with epoxy:
- Microballons (or microspheres) are tiny hollow spheres of glass (yes, really!) and they are a lightweight filler that just add volume to the resin.
- Microfibres are tiny cotton (or other) fibres and boatbuilders sometimes call them 'scrambled egg' as that's what they look (a bit) like when dry. Microfibres add strength and ductility to a resin mix.
- Silica (fumed silica) is a 'thixotropic' agent - it makes resin mix stiffer and more like a paste, before it cures. Trade names are Aerosil and Cabosil. The silica makes the resin mix after curing harder. Before mixing, silica looks and feels really light - a puff of wind will blow it away!
- Wood flour is dust from woodworking and is used to incease volume and to colour match to adjacent wood.
All of these fillers have specific properties and can be custom mixed to make a resin mix suitable for a specific job, such as:
- Just adding microballons will make a lightweight fairing filler that is easy to sand. If it was to be applied thickly in some places, a little silica (no more than 1:10 to the microballons) would make it stay in place better before it cures.
- Just adding silica will make a filler that is so hard as to be unsandable by hand when cured - only a sanding disc in an angle grinder will touch it.
- Just adding microfibres will make a superb wood glue - good strength and (unlike most strong glues) good ductility and impact resistance. Again, adding a little silica will help it stay in place before curing.
- A blend of equal parts of microballons, silica and microfibres makes an excellent filleting filler - easy to apply and to smooth, strong, just about sandable, and ductile.
Incidentally, I think it is the fillet that most non-users of epoxy need to learn about. Everyone understands the idea of a glue, and many understand the idea of a fiberglass-taped joint, but the use of an epoxy fillet instead of a strip of wood or glass to join two pieces of ply is very valuable, particularly where the joint isn't a simple right-angle. I'm just working on the Roswell design at the moment and the filleted joint is the only way to sensibly build that.
Here is a diagram of a fillet joint - my personal rule of thumb is that the leg length of the fillet should be 1.5-2 times the thickness of the ply.