At FGCI, we sell a number of fillers for resins and gelcoats. It’s hard to tell which one you will need and what the differences are. So, let’s go over the common ones so you can pick the filler that is right for you.
An important TIP is to make sure, when using putty, to not go higher than 1/8” thick on your putty without adding a layer of glass. This way any stress on the putty is held to 1/8” or less and any issues can be easily controlled.
With all of these different fillers, in order to get a frosting-like consistency, you will want about a 60 to 70 percent mixture to resin or gelcoat. The fillers are sold in pint, quart, 2-1/2 quart, 5 quart and box sizes for whatever job you need.
Chopped Glass – The filler is ¼” glass fibers. They are the biggest pieces of the fillers we have. The filler offers increased strength is good for corners and doesn’t affect pigmented gelcoat. This is the most popular for use with resin putty and is good for not cracking.
Cotton Flock – The filler is actually pieces of cotton. The filler promotes resin adhesion. It’s a filler that offers increased strength for the putty you make.
Fumed Silica (aka Cab-O-Sil) – The filler thickens resin with little change to the cured properties. It makes for a very creamy putty that is great for applying to a vertical surface. This is very commonly used to prevent draining.
Microballoons or Microspheres – Phenolic spheres are generally plastic or glass and are hollow, intact spheres that reduce the weight of the putty, and add stability. This filler is used for making fairing compounds. The phenolic balloons are the most expensive filler we offer. The Microspheres are more broken spheres and are generally smaller, making them cheaper, but not as light of a putty as the microballoons.
Milled Fibers – A much finer fiberglass than the Chopped Glass, it makes for a much harder putty. The putty is strong and very coarse.
Talc – This is a light, fine powder, generally for making bondo-like putty. It makes for a very smooth, easily sandable, soft putty. It is quite inexpensive, but does add weight and can absorb moisture.
Walnut Shells – A brown filler, favored for making a wood putty or sometimes used for non-skid due to the rather large particles. It offers good strength, but the color can make pigmenting the product difficult.