RB wrote:
A corner of my 18' cc o/b boat got knocked off. It's at a spot aft where
the gunnel, transom, and flat top side of the gunnel all come together. The
spot is approx 4" diameter.

I'm fixing to stick some auto bondo or similar on it and then mold/shape for
the right contour.

The question is how to be able to smooth it on with minimum surface
roughness. There used to be a number of tricks for doing that, such as a
paint stirrer paddle kept wet by dipping in water. Is this still the
easiest way to be able to soft mold the patch and get it close to where it
needs to be before I start shaping when dry?

If I need to, I can finish it with glazing putty, before painting.



Having read the other answers; it seems you're not going to re-glass the
area just cosmetically fill the hole & then reshape & finish??? I guess
you're happy there is no structural issue??? The deck to hull joint in
way of the transom &/or outboard well is fairly highly stressed??? Anyway,

(i) Use a polyester filler, it will adhere to freshly ground clean & dry
polyester fibreglass OK. The filler has little real strength & if the
area is structurally stressed & flexed it will crack etc.

(ii) Experiment a little so you get the amount of catalyst just right,
so it takes about 15 minutes before the filler starts to "gel" (you need
to experiment a little because the amounts of micro balloon fillers in
the resin can vary even from batch to batch & of course you need to
adapt to your particular temps humidity etc)

(iii) Do the filling in several stages, the first to well below the
final level needed & each time let the filler just start to gel then
shape it with a u profile surfoam blade (just hand hold it without the
blade holder it's light & easy to manipulate, because the filler is the
consistency of cheese or soap & will easily grate off in slices, if it
doesn't you've let it cure too long & no amount of "sanding" will ever
get a proper shape, so grind & re-fill) long smooth cross hatched
strokes & look along the boat, corner etc to get it right.

(iv) Once you have the filling & surfoam shaping up nearly to the final
level, take a plastic filler applicator & cut small but deep "V"s along
one edge (e.g.1/2" deep on a 1/4" base say 3/4" apart). Then apply a
coat of filler & carefully scrape it off with the V'd applicator. This
should leave you with nice parallel little ridges of filler. (the pros
put a bit of tint in the ridges so they're a different colour to the
other filler)

(v) Once the ridges have gelled & cured a bit so they're strong enough
not just to crumble, use a long board to sand in the "final" shape. Even
though the ridges are relatively hard compared to the cheese of (iii) it
shapes & sands very quickly & easily because you're only sanding skinny
ridges.

(vi) Once you're happy the shape is perfect then you can use a non V'd
applicator to fill in between the ridges, then after it's cured (unlike
the surfoam you can't sand filler if it's too soft, just clogs) start
sanding with an orbital sander (no courser than 80 grit) & here's the
trick, sand till you just start to uncover the tops of the final shaped
ridges & no further (that's why the pros use tint, they can see it
"through" the overlaying filler as they sand down).

(vii) After you have that then start surface finishing with filler
scraped into the 80 grips marks sanded off with say 120 then again
scrape filler hard over the surface into the swirl marks & sand till all
imperfections are gone.

(viii) If you want to use gelcoat to refinish then put it directly onto
the filler with that sanded surface, never ever try to put gel coat over
a primer or "paint" (where does he get this stuff???:-))

(ix) The gel coat will not harden on the outer surface (gelcoat has no
wax in it & you shouldn't ever add any either, but once you've put it on
& it's just started to gel cover it with cling wrap or even paint on PVA
water soluble release agent (this does the same as the wax & seals the
gelcoat from the air so it can cure full depth.

(x) Peel off the cling & sand with ever finer wet & dry, lots & lots of
water & always with a little sanding block (i.e. do not sand it with
hand held wet & dry, you will not get a good shine at the end)

(xi) Machine with cutting compound & if the colour of the gelcoat is a
good match you'll be proud of it.

K