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
-----Original Message-----
From: forum-owner@europaclub.org.uk
Subject: Surface area increase with sanding 41.4%
Duncan and others,
Surface area increase with sanding. Now I have been wondering about this for
ages. I realise sanding increases the surface area, but there must be a rule
which states what the maximum grade of paper to use to give the best result
i.e. you should use 80 Grit advised by Europa on layups before future
bonding
but it always seems a bit coarse to me. I only use 120 Grit for roughing up
an
already preexisting glass surface. How do you get 41.4% increase in surface
area?
Reg
Tony Renshaw
>On Friday, October 11, 2002 2:30 PM, Fred Fillinger
>[SMTP:fillinger@ameritech.net] wrote:
>
>> 180-grit is probably OK, if its use doesn't show through the coating,
>> but it appears that grit size bears little relationship to long-term
>> adhesion. The following is from a coatings manufacturer:
>>
>> "Although surface roughening generally improves the adhesion,
>
>
>Stits always advised that 240 was the coarsest possible without risk of
>show-through on the final coat; I found that to be correct (at least for
>his paints).
>
>The improvements in adhesion probably only comes about by the increase in
>surface area that the abrading generates. In which case there would be no
>benefit in a coarser roughening (i.e. if, for the sake of argument, you
>consider that a 90 degree saw-tooth profile is generated by the sanding
>then the depth of that profile does not alter the 41.4% additional surface
>area generated). Which is consistent with your comment.
>
>Nobody has yet mentioned the appallingly high % of talc that is in Smooth
>Prime. Apart from the poor adhesion of anything against talc, the presence
>of this would make wet sanding very inadvisable; it would be difficult to
>get it dry (and mineralogically un-hydrated(?)) afterwards, but not
>impossible with the correct technique.
>Personally, I added loads of Q-cell to the Smooth Prime, which makes it
>much cheaper and displaces some of the disastrously heavy filler minerals
>in there. My next coat was then a single-pack base coat; which is holding
>on okay so far.
>
>Duncan McF.
Reg
Tony Renshaw
Builder No.236
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