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System Three Filleting, Fairing, and Molding with Epoxy




The SilverTip Series contains two putty materials: SilverTip EZ-Fillet, a wood-flour filled putty, and SilverTip QuikFair, a microballoon filled putty. Neither involve user added fillers and powders. As described elsewhere these have other advantages beyond simply eliminating the use of obnoxious, dusty powders. We suggest that most epoxy users will be better off using these rather than whipping up a batch of "homebrew" epoxy putties. Once mixed SilverTip EZ-Fillet and SilverTip QuikFair are used as described below.

For those who choose to homebrew putties we offer the following: Our general purpose epoxy can be mixed with phenolic microballoons (purple), quartz microspheres (white), or wood flour (brown) to make a putty-like material that is used for making cosmetic or structural filleting, fairing, or molding compounds. The use of these materials with the right portions of silica thickener makes a smoother compound than the fillers alone can produce. The amount of these fillers is best determined by experimentation taking into account the desired results, temperature and viscosity of the epoxy being used. For previously stated reasons we neither recommend nor support the use of SilverTip Laminating Epoxy resin with fillers or thickeners.

Filleting is the process of adding an epoxy putty to concave angled corners for cosmetic and structural reasons. Cosmetic fillets are generally "low density" being made by the addition of microballoons, which "bulk out" the epoxy. Structural fillets are "high density" and are thickened with silica thickener, plastic thickener, or wood flour. These fillets sometimes contain glass fiber. Thixotropic agents make the mix non-sagging when sufficient amounts are used. Microballoons and microspheres do thicken the epoxy, but when used in proper loadings do not prevent sagging, and need the addition of a thixotropic agent like silica thickener.

Cosmetic fillets are applied by putting an excess of material along the length of the corner with a putty knife or caulking tube. Be careful not to force big air bubbles into the fillet when putting the putty into the corner. A rounded tool is used to shape the putty by drawing it along the fillet. The sides of the tool should touch both sides of the corner and the radius of the tool is determined by how rounded the finished fillet will be. Almost any material can be used to make a filleting tool. Plywood paddles work well, are easy to make and are inexpensive. The excess putty will be forced out on either side of the tool where it is scraped off with a putty knife.

Once the fillet has cured it may be sanded. A round edged sanding block with coarse (50 to 60 grit) paper works best. Knock off the high spots with the sandpaper and then come back and fill in the low spots with an additional batch of putty. This is much easier than sanding the whole fillet down to a common level. Blow or brush off the sanding dust (wear a dust mask!) Make up some more filleting compound and use a broad putty knife to fill the low spots resting the blade against the fillet parallel its axis. Allow the putty to cure and do a final sanding.

Before microballoons are painted they should be sealed with epoxy or else the paint goes into the tiny hollows in the broken balloons and the finish will appear ragged. Brush or roll on a coat of epoxy on the sanded balloons. Use either Clear Coat or SilverTip Laminating Epoxy thinned with about 10% denatured alcohol or lacquer thinner to make it easier to apply. Treat this cured sealer coat as any other epoxy coating before finishing.

Structural fillets increase the glue joint surface area relieving stress concentration zones that occur at angled corners. They are usually made at the same time that the piece creating the corner is attached. For example, when sheet plywood is glued onto a stringer the excess glue that oozes out can be used to form the fillet. A gloved finger makes a good filleting tool, as these fillets don't need to be large. Once the glue begins to cure it can be smoothed by rubbing with a solvent saturated rag. Wear solvent resistant gloves when doing this.

Large structural fillets are generally made in a separate operation in a manner similar to making cosmetic fillets. The addition of either milled glass fibers or chopped glass strands, improves the tensile strength of structural fillets.

Proper epoxy fillets don't need to be covered with fiberglass cloth. Apply cosmetic fillets after the fiberglassing is finished. This makes fiberglassing easier as the edges of the cloth can be run into the corners; left ragged, and then later is covered by the fillet.

Fillets in stitch-and-glue boat construction are usually fiberglassed. The easiest way to do this is to fiberglass the fillet when it is in a semi-stiff state so that it can still be pushed around with an epoxy-saturated brush. This saves having to sand the fillet after it has cured.

Fairing is the operation of filling the low spots on a boat hull or auto body to the level of the high spots, eliminating waviness and hollows. Use SilverTip SilverTip QuikFair for this or make your own. The compound used is identical to that of the cosmetic fillet and the operation is similar except that large flat areas are involved. Large wallpaper broad knives, stiff boards with taped edges, squeegees, and similar tools are useful for fairing. Once the putty has cured it is sanded with long sanding blocks to a level fair with the surrounding area. On very large areas low spots may appear during sanding that will need a second fairing. After final sanding the fairing compound should be sealed with epoxy prior to painting.

A slick way to fair a large area and avoid a lot of tedious sanding is to use a serrated trowel like the metal one floor tilers use to spread mastic. Apply the fairing putty using this tool leaving a series of parallel ridges that stand proud of the surface. Allow the putty to cure, and then sand the area with a long board. Notice that all you are sanding is the tops of the ridges, about one fourth of the total surface area being faired. Sanding dust falls into the valleys. Once the ridge tops are fair, the area is cleaned of sanding dust and the valleys are filled with fairing compound using a broad knife with a straight edge. Only a light sanding is then required for final fairing following cure. Seal with epoxy before painting.

Molding with epoxy compounds is a very useful technique that can be used to build winch pads, lifeline stanchion, pulpit pads, etc. A high-density compound like SilverTip EZ-Fillet should be used here. The idea is to make a pad on the hull or deck of the proper size and shape to mount the hardware. An example can best illustrate the technique. A six-inch diameter pad is needed to mount a winch. A plywood circle six inches across is cut and transparent cellophane tape is stuck all over it, to act as a release agent. SilverTip EZ-Fillet or a stiff structural putty of epoxy, milled glass fiber and Silica thickener is made and liberally applied to the taped plywood. The plywood is then located at the proper place on the cabin top and the puttied plywood is pushed down onto the deck. The plane of the plywood face is adjusted so that the winch will have the proper sheet lead angle. Tapping the plywood forces the excess putty out. When the plywood has been properly positioned, the excess compound is removed with a putty knife. The molded pad is allowed to cure and the plywood blank can then be knocked off with a hammer. Any voids are then filled with more compound and the pad edges are filleted with SilverTip QuikFair to fair them in with the cabin top.

Materials Required for Fairing, Filleting:

  • Epoxy Resin and Hardener(SilverTip QuikFair, SilverTip EZ-Fillet)
  • Spreaders
  • Microballoons /Microspheres
  • Sandpaper
  • Silica Thickener Fibers



  • This article and more can be found within The Epoxy Book



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