Working with Fiberglass Hulls

Some important information you should know before starting to work with
fiberglass hulls:
The hull beam out of the box may not match the true
scale beam of your model
Because fiberglass is a flexible material, and due to the nature of the molding
process, most hulls will need to be either drawn in or spread out during
the construction process.  This is easily done by installing deck beams
of the correct width.  See the page on
installing decks
for more information on attaching these.  The length of the ship should
be accurate out of the box. Always check your beam against your plans every
six inches along the model length.  This is vital, as an errors at this
stage will cause tremendous problems later.
Check the deck edge height of the hull when it is

This is important, as it can cause the main deck to be set incorrectly.
 Although not a common flaw with commercial hulls, it can be caused
my inaccurate trimming of the hull while it is in it's gel stage.  
It is easily checked using a waterline marker:  Find a level area on
your workbench, and shim the hull so that it is level side to side and front
to back.  For some models, thin wooden wedges work well for this, for
others clay pressed into place may do the trick.  Once you are satisfied
that the hull is level, you can run your waterline marker against the deck
edge and match it to the plans.  At this point it is very easy to build
up the hull or trim it down if needed..  Trust me, it is much harder
later - as I found out once!
Waterline marking tool.  Click to picture to enlarge.

Sand the hull anywhere  you want something to stick to it

Waxes and release agents help to get the fiberglass hull out of the mold, and are also need for the interior to cure properly.  These same products will prevent paint or glue from forming a strong attachment to your hull.  I have seen folks use very strong epoxy to attach motor mount and prop shafts - glue that popped off the hull with only a moderate impact.  For glue, body filler or catalyzed putty, sand with 80 grit sandpaper in the area(s) you wish to adhere.   A dremel tool with a HSS cutting bit can also rough up the area sufficiently.   You do not need to grind or sand deeply, just enough to make sure that all the surface you are working is cut.  You will see what I mean the moment you begin on the interior.

For the gel coated outside of the hull, you should sand the hull with 220 - 320, depending on the amount of work you need to do - well made hulls can use the finer grades.  You should sand all the shiny areas of the hull so that it is uniformly dull.  A wipe down with lacquer thinner or alcohol will leave a clean surface ready for primer.  Sanded body primers of the type sold in automotive stores work very well as a primer for fiberglass.

Tape the hull before drilling into it

The gel coat used by many manufactures to provide an attractive and smooth outer surface can easily chip when being drilled.  To reduce or eliminate this, use a transparent tape over the area before drilling, filing or grinding.  I use Scotch brand magic transparent tape for small areas, and thin clear packing tape for larger areas.

Fixing cracks and gouges

If you get a crack in the gel coat, or small gouge, this is easily fixed.  The important thing to do is take a small ball shaped Dremel tool bit ( you can use a file if you don't have a Dremel), and widen and deepen the crack..  Although this sound counter-intuitive, this gives a greater surface area for the polyester glazing putty to adhere to, resulting in a stronger repair.

Fixing larger areas

Hey, sometimes it happens.  You drill a hole where you should'nt, or drop a hull, something else catastrophic happens to it.  Not to worry - it can be fixed.
Put clear packing tape on the outside of the hull, trying to follow the curvature of the ship as closely as possible.  Grind out the interior a surface around the repair so that is at least 3 times the total square area of the damaged section.  Lay a piece of medium (or heavy if a very large repair is needed) weight cloth over the entire area, but make sure it does not extend beyond the ground sections.  Mix up a batch of fiberglass following the manufacturers directions.  Using a disposable brush (acid brushes work well), stipple the fiberglass onto the cloth to tack it into place.  Make sure you work out the area under the cloth where it lays against the ground section of the interior hull.  If the area is large - say over 1 square inch, another layer may be needed - use the thickness of the hull as a guide for how thick you should build up your repair.
Once the fiberglass has dried (overnight, faster in warm weather, slower in cool) , you can peel off the tape, and finish the bottom repair with catalyzed body putty as needed.   From there, sand and paint normally.

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Version 3.0  4-19-20