Warning: If you glass the "outside" of the hardboard it will require a
lot of time to yield a nice, smooth finished surface. First of all,
multi-faceted surfaces need to be laid up in multiple passes, because it
is hard to cover an outside corner without creating bubbles (surface
tension is your enemy when laying up fiberglass). With your turret,
minimally 3 lay ups would be required. Second, after the lay ups,
you'll need to sand/fill multiple times to get a smooth, metal-like,
surface.
Personally, I would use the hard-board to make a one-time negative mold
and then lay up the fiberglass inside the mold form to make an
all-fiberglass part. The result will be lighter, will provide more
interior room, will be completely waterproof and will require less
sanding on the finished surface.
Basically, just make your hard-board box so that the "interior" surface
of the form is the desired "exterior" measurement of the turret.
Hard-board is great for molds, because it is easy to work with and has a
nice smooth finish. For flat sections, place reinforcing ribs on the
outside as needed so that you can press on those sections when laying up
the glass without them flexing. Tack nail or glue them in place.
After the box is built, coat the inside with a couple coats of sanding
sealer, with a very light sanding in-between each coat. Then, rub on a
couple coats of paste wax to seal it completely. Finally, spray two
light coats of PVA on the surface, allowing it to dry in between coats.
PVA forms a water-soluble film that makes it easier to release the
part from the mold. Your turret is simple enough that you might not
need PVA, but it never hurts to use it.
For lay up, I always use a layer of "mat" against the mold, because it
yields the best outer surface (no cloth pattern). Then, another layer
of mat to build up the thickness and finally a layer of cloth to keep
things neat on the inside and add more strength. That's plenty strong
for our purposes, especially given the structure of the turret. If you
think flexing will be a problem on a flat section (like the top), use
carbon fiber cloth in those areas to stiffen them.
After the part has cured to touch, slowly pry it away from the mold, a
little section at a time and it will eventually pop out. Wash off the
PVA residue with water, trim the edges and you should have a perfect
turret. You can then cut whatever holes are needed.
If you've never done a mold before, make a smaller version and learn on
that. It's not as complicated as it sounds. And, once you've mastered
molding, your 3D printer will become the perfect mold-making tool.
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