First of all don't use mat with epoxy. The binder that holds mat
together is designed to desolve in the styrene in polyester. It will
not bond to epoxy. Even if it did, mat is primarily for adding bulk and
is terribly inefficient for carrying the loads in a strip composite
hull. Loose thick "Roving" also is less than desirable with epoxy. It
takes to much resin to fill adding little to the strength and a lot to
the weight. Uni-directional and bi-directional knitted fabrics are
used to match the strength of the epoxy. Two layers of DB180
bi-directional is about 3mm. Two layers of A-130 unidirectional is
about 1.5mm.

With that out of the way, vacuum bagging is definitely worth if if you
want the most strength with the least weight. It is not all that hard
to do. Check out my web site to see how a team of 6 volunteers with no
experience glassed and bagged my 45' hull.

Fairing putty on a well laid hull ranges from none to what ever is
necessary to get a fair hull. It is not for filling the weave but for
evening out the overall lines. Normally when lofting station molds you
don't consider the fairing putty thickness. If you do everything
exactly right you won't need much putty. If you deliberately allow for
it you will be forced to use a lot more than you would otherwise.

Pete wrote:
On average, how thick is the putty fairing layer over the mat on a
strip planked hull? I guess it may depend on whether vacuum bags were
used (that's a question too - what's the general consensus on vacuum
bagging a hull?; what are the gains 'cos it looks like loads of
work?).

I'm just trying to cut the molds from the designers lofting templates
and I want to know how much to reduce the size of them by, to allow
for the overall hull thickness. But I also need to order the right
amount of filler too.

The glass suppliers have told me that two layers of 500g m² mat of uni
directional rovings laid at 90° add up to about 0.75mm after layup -
sorry about the metric measurements for the Americans, I haven't a
clue how many ounces/yd² that is - which sounds negiligble to me, but
I have read somewhere of around 3mm or just over 1/8 inch may be
fairing filler.

This sounds a lot to me (after my many attempts at plastering over
defects, I feel that the more you put on, the worse things get). It
seems to me that if easily sanded fairing is used it must be a bit
soft, so would the minimum necessary be used?

I'm talking epoxy here BTW, and I'm assuming that I will make a
reasonable job of laying up the mat .............. hmmm, I can hear
the gods of Famous and Doomed Last Words stirring from their slumber
already.......

TTFN
Pete

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com