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Not sure if it is called filleting or fairing, but I know nothing about fiberglassing. I have a weekender. I am almost done final sanding, all screw holes, imperfections, etc. are filled in with an epoxy puddy and sanded down. However, I know I have to do something with the 90 degree angles in order for the glass to adhere. I bought raka brand epoxy with wood flour, but I am picturing the stuff to "sag" and picturing having a nightmare grinding down these areas to get it smooth. Any advice would be greatly appreciated. By the way, I am assuming that ALL 90 degree angles require this? (keel/bottom joint, seat/seatbacks. seat/bulkhead, etc). Any other prep work required?

Hey Jeff,
Good to hear your build is coming along.

Filleting goes on the inside of a 90* corner, just like when you caulk a bathtub. You're right in that the glass won't stay 'stuck' in a sharp corner.When you mix the filler into the epoxy, keep it just thick enough that it'll hold a shape, like thick whipped cream. I find it's easiest to work in short sections, doing filleting and glassing at the same time - that way you're putting wet cloth onto wet 'putty' for a good bond (and no grinding!). You'll need it on any inside corners that are being glassed. Even if you're not glassing, it does finish it off nicely if you're going to paint.

Fairing is done with the same ingredients (more filler) mixed into a lightweight putty that's easy to sand when applied over imperfections. That's what you did to 'fair' or 'fair in' the screw holes.

That being said, I haven't built a Stevenson boat yet so I can't speak about particular spots on these boats.

For a good read, check out the West System User Manual. It's a good epoxy 'primer' (Sorry... :lol: )

A fillet is a pile of epoxy with reinforcing filler added, that "fills" an inside corner, so it isn't a hard 90 degrees, but radiused. This forms a flange which greatly increases the strength of the joint. If 'glass cloth is applied over the fillet, this too dramatically increases the strength of the joint. Making fillets is a bit of "feel" which you'll develop after several feet of applying the goo. Shoot for a nice smooth radius, tapering out to nearly a feather edge. Applying tape on either side of the fillet area prior to epoxy, can make for a very neat fillet and speed up clean up.

Fairing is the act of smoothing a surface. This is cussing and beer drinking time of the job. There just isn't much Zen in sanding and long boarding a surface, just elbow pain and the joy of finding yet another low spot needing filling. You can easily go quite insane trying to make a hull America's Cup contender smooth. Eventually most just get to a point and say "it's good enough, damn it".

Often fairing requires the surface imperfections be raised up the level of the surrounding area. So you'll smear on so thickened goo and knock it down. Other areas will be high and you'll sand them down until they're flush with the surrounding surface. It sounds simple and there are many tricks that can help, like a light dusting of contrasting primer to highlight the low spots. In reality, it's a lot of work, particularly if you used less then smooth materials or have lots of divots to repair.

Log onto www.westsystem.com and have a look see at their "how to" sections. They'll send you free booklets of all the techniques and product descriptions, which can be handy to the beginner.

I understand the what and why, I guess I am not sure of the "how". I can't picture mixing the epoxy and wood flour to a consistency that would "stick" in place without sagging, drooping, or running. Maybe if you put enough wood flour in it will work???

I used bondo per the instructions but then I didn't use epoxy. There may be a problem there.

Using fillers, mixing and handling epoxy is an experience driven set of skills. You can learn a lot from books and web sites (West System has videos), but in the end, you'll only feel comfortable after you've "smoked" a pot or three.

Log onto my site and look over the epoxy sections (Liquid Joinery, Epoxy Tips and Tricks and Painting).  >  www.PARyachts.blogspot.com  <

There's a wide variety of fillers, each with different properties that are added to the epoxy resin. Two primary types are used, adhesive reinforcement fillers and fairing compounds. The adhesive types strengthen the joint, with some being very hard (dense) and others less so. Certain types of bonds will require different fillers. Bonding stainless hardware to a 'glass cover deck for example, compared to a joint where a piece of end grain is involved. The end grain joint will need a "wetter" mixture, that penetrates well and usually with some fibers in it to help the bond. The hardware bond will just require a dense filler material, usually pretty thick that will bridge the gaps.

In most filler applications, you'll use more than one type of filler. An example would be needing a strong adhesive bond on wood to wood joints, but one that doesn't run out of the joint as it cures. I'd use some West System 404 (a high density filler) and some 403 (micro fibers). The 404 makes the joint very strong, but the mixture will tend to sag, so I'd use some 403 to prevent this and create an interlock between the surfaces, because of the fibers in it.

Cab-O-Sil is a product many use as a catch all for their adhesive filler needs. I use it sometimes, but usually mix up my own recipe, depending on what I need the bond to do.

When it comes to fairing compounds, you'll almost always have to mix some thickening agent into the fairing filler. Micro balloons (or spheres) are the typical fairing fillers used. They're literally microscopic hollow balls of 'glass or polyester. This creates a very light weight mixture that is easy to sand, just what you want for fairing large surfaces. It will sag to all get out if used alone, so I'll toss in some silica (West 306) to thicken it up and prevent the sagging. Properly mixed, using the right materials, you can apply thickened epoxy directly overhead and it will not sag, move, drip or run. This takes practice and experience to get right each time you mix a batch up.

I do it by eye and feel as I'm mixing it. I can judge how thick and what material to use from my experience and the "feel" it has as I mix. Some of the epoxy manufactures offer "mixing" guides, which have ratios of different materials for the task. Other manufactures offer pre-mixed resin, which has the fillers in it. This is a costly way of buying epoxy, but can save the trouble of figuring out how to work with fillers.

Generally you'll need enough filler to make the epoxy to at least a "mayonnaise" consistency in a fillet. This will cling to vertical surfaces, but may "creep" slightly during the cure. If you have concerns about this sagging or creeping, then mix it thicker until it's a "peanut butter" like consistency. This will stay put and not sag. As a rule, silica is used to control viscosity, so it doesn't sag or run.  You can tell by poking it with a putty knife and pulling up a small chunk. If the thin curly cue on top of the chunk falls over after your knife comes out of the goo, then it's too thin to resist sagging. If the goo "stands" up when you pull your putty knife up through it, then it's thick enough to apply vertically or overhead without worry. You want to error on the too thin side, rather than too thick. You can always add more, but you can't remove any if you've added too much.