On Sun, 11 Apr 2010 11:13:54 -0700 (PDT), wheewhoo
wrote:

Hi guys, thanks for the replies.

So what I need to do is sand off the gelcoat (and whatever other
layers are there), as I can definitely see the fiberglass where flakes
have come away.

Do I just need to prepare the bare (fiberglass) surface, and apply a
new gelcoat? Or do I need layers of other stuff also? Lets say I have
a large area of hull/keel that I have sanded back to the fiberglass.
What are the steps I should take? Phantman mentioned bottom paint, and
Bruce mentioned epoxy. Would it be epoxy, then gelcoat, then bottom
paint?

Thanks for the help!

The basic task if to protect the underlying fiberglass from UV damage
(a real threat in tropical climates) and water ingress which may take
place over a longer period of time. There are two basic methods, to
apply a gel coat or to paint, usually with an epoxy based paint.

The gelcoat is much harder to apply and have a nice smooth shiny
surface. The paint is much easier to apply but perhaps not as long
lasting.

Gelcoat Method.

Grind/sand, the damaged areas back to a good solid surface. This
surface can be either fiberglass or gelcoat but it MUST be a good
solid surface. If gelcoat you must be certain that it is attached
firmly to the underlying fiberglass.
Once you have reached your base surface you must then "fair" the
surface as smooth as possible as any craters, dips humps, etc. on the
surface will be mirrored in the final finish. You can do this by
grinding/sanding the surface until it is flat or you can build up the
low spots with an epoxy filler using any of the harder, stronger
fillers.
Once things are fair you then sand with varying grades of sandpaper to
about a 500 grip sandpaper.

Gelcoat is nothing more then polyester resin with a coloring agent in
it and comes in many colors. The mix also has a "wax" incorporated in
it that when it is applied floats to the surface and causes the
polyester to harden with a non-sticky surface.

The gelcoat can be applied with a brush or by spraying but no matter
how it is applied will probably not be the smooth, shiny, beautiful
coating that you have envisioned. It needs work.

Sand the gelcoat with progressive grades of sandpaper down to at least
1,000 grit and preferably finer. Once you have the surface as smooth
as possibly then buff the surface to the required finish using a
buffing wheel and polishing/buffing compounds as used for paint.

If you did a good job it will be beautiful.

Painting - Basically you do all the preparatory steps to get the hull
smooth and fair and then apply two coats of epoxy resin to seal the
hull. One method to avoid the "epoxy blush" problem is to apply the
second coat of resin before the first coat is completely hardened.
When it is no longer sticky but not hardened completely so that you
can press your finger print into the epoxy apply the next coat.
Finally after the last coat of epoxy (before it completely hardens)
apply at least one coat of epoxy high build primer.

Sand the hardened primer with about 400 grit sandpaper and do any
touching up, sand again if necessary, and spray two coats of two part
polyurethane paint.

It will be beautiful if you did a good job.

But don't kid yourself , either method is a substantial amount of
work. The gelcoat considerably more work, but neither is a Sunday
afternoon project.


Cheers,

Bruce
(bruceinbangkokatgmaildotcom)