Refinishing gelcoat

Why aren't we doing this. It seems that you could DA sand
the wings in a few hours, shoot 200 bucks worth of primer,
sand the primer in a few hours and be ready for topcoat.


The issue of removing the old Gel Coat is generally a much bigger job
that most people realize the Gel Coat the quite hard and the composite
underneath is generally realtively soft and thin. So once you cut
through the Gel Coat the DA tends to quickly cut through the 1st
layer(s) of the composity very quickly weakening the structure. So
usually while a DA is carefully used to remove the majority of the
Gelcoat the last 10% of of it need to be carefully removed by hand
sanding to avoid damaging the composite.

Secondly those cracks in the Gel Coat as the get worse will transfer
into the Composit actually causing cracks in the Composite structure.
When removing the Gel Coat this can easily be seen as small white
cracks in the Epoxy itself. I have seen one aircraft that the 1st
layer of fiberglass had to be replaced because on all the cracks in
the fiberglass.

Even with Gelcoat the Materials are only a very small fraction of the
cost. The Majority of the cost is labor of removing the old Gel Coat.

Brian

Historically we have shown in the composites industry that you can't
effectively "fill" gel coat cracks with another coating, or simply cover them
up. In examining the mechanism of gel coat cracking two issues become apparent:
In the case of hairline cracks, the inherent viscosity and surface tension
characteristics of most coatings will not allow them to penetrate the fracture
area. If the crack is large enough that a coating will penetrate the fracture
cavity, the bond strength between the coating and the crack sidewalls is not
sufficient to prevent the development of two new micro-cracks on each sidewall.


The only effective method of preventing crack propagation is to remove the
crack in it's entirety by sanding to the bottom of the fracture area.

The use of thermoplastic shrink film over a gel coated surface is an intriguing
idea. Tedlar, in particular, performs very well in long term outdoor exposure.
However, keep in mind that it would mask the cracks in the gel coat that
potentially could propagate into the laminate substrate. I would have the same
level of concern about hidden cracks in composites as I have with hidden
corrosion in metal aircraft.

Bob Lacovara

"B Lacovara" wrote in message
...
Historically we have shown in the composites industry that you can't
effectively "fill" gel coat cracks with another coating, or simply cover
them
up. In examining the mechanism of gel coat cracking two issues become
apparent:
In the case of hairline cracks, the inherent viscosity and surface tension
characteristics of most coatings will not allow them to penetrate the
fracture
area. If the crack is large enough that a coating will penetrate the
fracture
cavity, the bond strength between the coating and the crack sidewalls is
not
sufficient to prevent the development of two new micro-cracks on each
sidewall.


The only effective method of preventing crack propagation is to remove the
crack in it's entirety by sanding to the bottom of the fracture area.

The use of thermoplastic shrink film over a gel coated surface is an
intriguing
idea. Tedlar, in particular, performs very well in long term outdoor
exposure.
However, keep in mind that it would mask the cracks in the gel coat that
potentially could propagate into the laminate substrate. I would have the
same
level of concern about hidden cracks in composites as I have with hidden
corrosion in metal aircraft.

Bob Lacovara


If the 'process' was to first remove the cracks by sanding back to the
substrate, then fill and flat locally with a polyester filler and then to
apply film to the entire surface then perhaps we might have a solution?

The biggest time consumer I find is the manual flatting and polishing
required with either Gel or paint finishes.

Ian