I think I didn’t order the milled glass fibers. This not the same as micro balloons right?
No, not the same. There are three types of fillers for epoxy - thickeners, strengtheners, and fairing compounds. They all have different properties and are used for different reasons. Here's a run-down on what's what:
Thickeners (wood flour, colloidal silica / cab-o-sil) create a higher density cured product that's good for fillets and general gap-filling or gluing purposes. Examples: Most fillet for seams that have glass over them will use the 80% wood flour / 20% silica mix since it's higher density, fillets smoothly, and doesn't sag ... noting that wood flour thickens, but will sag while curing (like putting pudding on the wall), and silica thickens and does
not sag (like toothpaste .... soft but doesn't sag). Adding silica to wood flour while thickening epoxy makes a thickened epoxy that's cheaper to make, e.g. versus using all silica. The wood/silica blend sands a little easier too, while silica cure to stone-like hardness. Sometimes only little thickener is used if you
want the mixture to flow. An example of this is the use of silica alone to create a slurry-type mixture used for laminating layers of plywood together (you want the broad flat surface between 2 layers of plywood to be glued well everywhere - good coverage) or for gluing up things like scarfs so that the mixture is both gap-filling and flows well enough to fill the joint.
Strengtheners (milled glass fibers, ground glass fibers, linen fibers etc) are used in addition to the thickeners above. They create a mix that is harder to crack .. the fibers 'tie' the epoxy together sort of like epoxied fiberglass. Strengtheners are used for keel, bow, and transom corners because these area experience more flex and stress (corners are stress concentrators) than other seams. Some people will use a softer, more slurry-type, mixture with strengthening fillers in it for gluing up scarfs but if the scarf joint has fiberglass on one side or the other or both, then this isn't necessary - but never hurts.
Fairing Compounds (microspheres (glass), microballoons (phenolic), plastic fibers etc) are designed to be lightweight and easy to sand. Generally, these are mixed (alone) into epoxy just enough to make a soft paste or creamy-textured mix for filling surface defects or to smoothly hide the edges of fiberglass etcetera. Once cured, you can sand or long-board (sand) to a fine finish prior to adding a final coat of epoxy and then painting the boat. An exception to this is the phenolic microballoons - microballoons create a mixture that easily sags and must be mixed together with just enough silica to make it non-sagging. Too much silica will make it hard to sand. Phenolic microballoons (dark purple) are microscopic 'balloons' or hollow spheres of phenolic. Glass microspheres (white) are microscopic 'balloons' as well, but made of glass instead of phenolic. Plastic fibers are what they say they are, but reduce the density of the epoxy to make it easy to sand. In general, from finest to least fine fairing compounds, they are: Plastic minifibers (will fill fine sanding lines), glass microspheres (sands to a fine edge), and then phenolic microballoons (sands to a fairly fine edge).
An
aside: If you select a high-body paint primer, then it acts as a fairing compound as well .... a thick primer can hide a thousand sins. If you use the fairing compounds above first, then add a final coat or two of epoxy (sand with fine-grit paper after each), then use a high-body primer ... you'll get a nice looking boat
Mixing - I mix strengtheners in first (milled glass fiber) since they seem to mix in the easiest if you do, then I add silica by eye, then I finish with wood flour and add just enough to get the thickness right. For the fluffy stuff like silica, I generally step outside the shop and hold the cup away from my face while I mix the silica in ... even a slight breeze will move the particles away from you - just turn so that they don't blow
on you. Once the silica is in, then I go back inside the shop to add the wood flour. I hate wearing face masks.....
Here ... browse the product descriptions here:
http://www.fiberglasssupply.com/Product_Catalog/Fillers/fillers.htmlHope that clarifies fillers a bit....
Brian
