Scott,

The short answer is - the filler products I use are epoxy/woodflour, epoxy and epoxy/phenolic microballoon mix.

The detailed answer is:

The outside of your boat is made up of several layers, each with its own function. For a typical CLC boat, starting from the inside there's
    1. the wood which makes up the actual structure,
    2. a layer of epoxy to seal the wood against water and provide abrasion resistance
    3. an optional layer of epoxy-saturated fiberglass that provides strength and/or abrasion resistance,
    4. some kind of fairing material to smooth the surface
    5. the finish surface, either paint or varnish.

If layer 3 is applied, layer 2 may be skipped as an explicit layer and be provided as a side-effect of layer 3.

Each layer provides an important function, therefore each damaged layer needs to be repaired. So the first important thing to determine is how deep does the scratch go. The other important factor is how wide is the scratch. If it's very narrow, you can get away with a temporary (good for the season) fix of just packing it with a filler, sanding it smooth and finishing it, even if it's deep. The final factor is how much you care about the cosmetics. Are you trying to get it to look brand new again, are you going for a yacht finish or are you happy with the look of a well-used and beloved old boat?

If it's a deep gouge in the wood, the wood needs to be filled with something like woodflour and epoxy putty and sanded smooth. In that case, especially if the gouge is wide, I'd also put a fiberglass patch over it, even if optional layer 3 wasn't originally there. That's because wood gets its strength from the lignin fibers that run its length and a blob of putty doesn't have that. If it's a shallow scratch in the wood, painting the affected area with epoxy will be OK as long as the area is not under severe tension from being bent.

A small fiberglass scratch in a low-stress area can also be fixed by painting it with epoxy. As more glass fibers are damaged, eventually you'll need to remove the damaged glass and apply a patch.

For varnished CLC boats the fairing layer is epoxy, so fixing that layer is simply wet-sanding off the existing varnish from around and over the damaged area and then applying new epoxy and varnish. Same for scratched varnish, just without applying epoxy.

For painted boats, I like to fix layer 4 with epoxy/phenolic microballoon mix. It's very easy to sand and takes paint well, even without primer.

I've left out any discussion of technique, so be aware that it's not as simple as just slapping the product into the scratch/gouge. There's a lot of surface prep, cleaning and sanding and finish work that I've ignored here.

Good luck,

Laszlo

   Thank you for such a detailed post. This is exactly what I'm looking for. Fortunately I don't have any deep scratches.

Rocks abound in my area.  For my Skerry, which has a painted hull, scratches are inevitable.  I use Bondo Lightweight Filler and then paint over it when necessary.  It's a two-part polyester resin that doesn't shrink and can be found in any auto parts store.  My boat is still looking great and coming up on 20 years (jeesh, hard to believe that).

Keep in mind that Bondo, as all polyester resins, gives off styrene gas as it cures. As a result, it is porous. So you should always seal it with at least a good coat of varnish or paint.

Laszlo