> We are in the process of finishing our Europa with the Poly Fiber System.
>
> We also had problems with the Top Gloss sticking to the primer on our test
> pieces but found that we were producing too smooth a surface on the primer.
> We had to roughen it up with 180 grade sandpaper. Found it impossible
> to peel it off after that - - but - - we have not flown it yet !!
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, care
must be taken to avoid deep and sharp profiles. These types of
profiles allow the formation of non-uniform films, which act as stress
points that tend to reduce the coating durability by weakening the
adhesion. As long as the films are somewhat fluid, the shrinkage,
uneven depths, and three-dimensional changes introduce little
unrelieved stress. As viscosity and film stiffness increase and as
film adhesion to the substrate develops, substantial stress are
accumulated and retained in the dry film."
Elsewhere they state that merely a "weathered" paint surface - no more
gloss - is sufficient for tooth adhesion. Also, where the above cites
viscosity, note that polyurethanes tend to be the least viscous, IOW
stiff and hard.
Regards,
Fred F.
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