Ideally, only the epoxy on the surface of the layup wants to be abraded.
Any deeper results in removal of structural material. But that's probably
all theory as the important thing is to generate a clean high-energy
surface; the resulting improvement of bond strength would likely outweigh
any loss of glass, given that the weakest link in the chain is in most
cases going to be the bond, not the glass.
Of course there comes a point ....................
To obtain a good paint bond in car paint refinishing, it is sufficient to
just dull the substrate surface using matting compound.
41% comes from simple trigonometry.
Duncan McF.
On Saturday, October 26, 2002 12:24 AM, Tony Renshaw
[SMTP:tonyrenshaw@ozemail.com.au] wrote:
> 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
|