I thought this would be of interest to the group. Gary is a very
experienced homebuilder and thought of as US chief epoxy wizard.
Graham
[The Canard Aviators's Mailing list]
John Bullens wrote:
... talked to the PPG tech rep and was told to spray a top coat
because none of the PPG fillers or epoxies has U/V blockers in them only
the top coat offers U/V protection.
The PPG tech rep is correct about fillers and primers not having U/V
blockers. They don't need them - the top coat protects them. One must
understand, the U/V blockers in ANY top coat paint are not there to
protect your fiberglass or primers underneath it. The U/V blockers in
the top coat are for the benefit of the TOP COAT ONLY. In fact, just
the first few microns of a color top coat, and the full thickness (2
mils or so) of a clear coat. The purpose of the UV blockers in a top
coat is to enhance the weatherabillity of the top coat, thus it keeps
its gloss and beauty much longer. It is the pigments in the color top
coat that blocks the UV from reaching the primer. This is not to say
the UV blockers in the top coat do not contribute to the "overall" UV
blocking capabilities of your paint job. I am simply saying - it is not
the original intent OR sole purpose of the UV blockers that might be in
a top coat formulation.
With regards to UV protection of your fiberglass - It is the PIGMENTS in
the primers and paints that blocks the UV light from reaching the
fiberglass. And as such, so long as you have a GOOD even coat of any
type primer on your fiberglass, you will have adequate UV blocking for
your fiberglass and foam underneath.
SO - I can tell you beyond a shadow of a doubt - you can safely fly your
airplane in primer without fear of it disintegrating in the sun. The
primer won't last 5 or 10 years like a top coat - but it will do just
fine for a year or so.
With regard to "pin holing, cratering and crawling" of the top coat
applied after having flown the airplane in primer. One bad experience
does not make a rule. There are many reasons for these problems to
occur. Most of the time, it is inadequate surface cleaning - PRIOR to
prep-sanding the primer for top coating. A thorough detergent washing
is a must to remove surface contaminates - most significantly - oil.
If you do not do this, all the sanding in the world will not remove the
contaminants. It simply pushes them further and further down into the
(now cut) surface of the primer you are sanding.
So, BEFORE prep sanding, - very thorough detergent washing(s) with
thorough water rinsing(s) - then follow the recommendations of the
coating manufacturer to the "T".
One more note - It should be a light colored primer. tan, white,
light-light gray, - NO blacks, or dark grays or greens. PPG makes a
white epoxy primer. You can use any color filler/primer filler etc.
But, if they are dark, you should at least use a white epoxy primer for
the final primer coat prior to flying.
Gary Hunter
Shell Chemical Co -
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