Your useful tests confirm the theory.
Otherwise, although the water test is common, I'd be reluctant to use it
other than for developing a cleaning procedure. This is because the
presence of even small amounts of atmospheric moisture have a significant
effect on epoxy bond strengths; reduction of atmospheric humidity at the
time of bonding results in proportionate increases in bond strength, down
to an RH of 5 to 10 % where the benefit flattens off. So if the water used
in the water test is not fully dried off (hairdryer?) it will result in
reduced bond strength.
I don't think we should get too paranoid about this because Europa have
allowed generously in their design. They may correct me if I'm wrong, but
the design-allowable bond strength (500psi) is about 25% of the maximum
that can be achieved with these materials.
Duncan McF.
On Saturday, October 26, 2002 3:17 AM, Fergus Kyle [SMTP:VE3LVO@rac.ca]
wrote:
> Cheers,
> On the subject of sanding and 'substrate' I think we have to
> consider hwat type of surface we're discussing. I've been scuffing old
> epoxy-glass layers, fuselage new outer surfaces, long-open peelply and
> alumin(i)um flats at various times. I started with sixty, but soon
> discovered I was ripping and tearing. then I read somewhere that Boeing
says
> "if water will stay on it, it's ready for sticking" - or words to that
> effect.
> Since then I've treated several different tests surfaces (as
> above) with a number of different grades of abraders and frankly I found
the
> BiD that sticks the best is to ANY of the surfaces that have been treated
> within the hour to a palm-sander from 120 to 320 grit, smeared very
lightly
> with epoxy compound and applied with pressure, but not a lot (weights).
The
> latter were the hardest to pull apart after 6 hours, two days and two
weeks.
> Much as I dismiss Boeing - the engineers who build aircraft
for
> profit - I'm going with the water theory.
> My CAN$0.02
> Ferg
>
> ----- Original Message -----
> From: <jeremycrdavey@btinternet.com>
<forum@europaclub.org.uk>
> Subject: Re: Surface area increase with sanding 41.4%
>
>
> > I'd be most interested to know what other builders have used. I've used
60
> > grit for the scuff sanding inside my XS wings, but have noticed that
the
> > sanding marks adjacent to where the rib-to-spar joints have been made
> during
> > manufacture appear courser - 40 grit?
> >
> > I find myself constantly torn between the risk of not sanding enough
(and
> > getting a poor bond) and overdoing it (and damaging the substrate).
> >
> > Regards,
> > Jeremy
>
> >
>
>
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