Agreed. But it also depends on the depth o any "contamination". I have no
clues on that, only it is probably very small given that the permeability
of epoxy is low.
Duncan Mcf
On Saturday, October 26, 2002 6:50 AM, Nick Hammond
[SMTP:Nick.Hammond@saabsystems.com.au] wrote:
> All,
>
> I remember reading somewhere (but now can't find it) that the point of
scuff sanding existing surfaces was to improve molecular bonding between
the surfaces rather than simply to increase the surface area. Apparently
breaking molecular bonds in the existing surfaces encourages some molecules
to re-bond with the new epoxy. On this basis, (since molecules are small) a
finer grit would be better as it results in many small scratches rather
than fewer large ones.
>
> Best regards,
>
> Nick
>
> -----Original Message-----
> 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.
|