Advice Needed on System 3 Clear Coat for Cowl Pinholes

At the stage now of finishing my RV-7A cowl.

For those that have used System 3 epoxy Clear Coat for pinhole filling, any advice on how to apply it, number of coats, time between coats, sanding, priming afterwards, etc.?

These guys know pinholes....

http://www.canardzone.com/forum/topic/16536-cory-birds-raw-epoxy-finishing-technique/

Try your epoxy on a scrap in very thin, as in squeegeed off, coats and see if it cures nice and hard.

flyingriki said:

http://www.canardzone.com/forum/topic/16536-cory-birds-raw-epoxy-finishing-technique/

Try your epoxy on a scrap in very thin, as in squeegeed off, coats and see if it cures nice and hard.

Click to expand...

Found this in the canard archives. Written by a Grand Champion Lancair finisher. Same as the Cory Bird Technique as DH has often suggested to those that listen. Use West system and don't dilute more than recommended by the manufacturer.

http://curedcomposites.netfirms.com/finish.html
Finishing a composite airplane
Foreword
I wrote this because I see a lot of people struggling with finishing their airplanes and I don't see
anything well written on the subject anywhere. I have seen a number of articles in magazines and
books but they all seem to be written by people who are writers rather then builders and never
actually did the job, let alone would be good at it. The latest example of such article is in Sport
Aviation 2/2003 named The Joy of Sanding, four pages filled but nothing helpful about how to
do the job. I, on the other hand, am no writer and English is a second language for me to top it
off, so bear with me. My own experience comes from building composite tools and production
parts for kit airplanes as a business and from doing all the finish work on an award winning
Lancair IV P for Mike Shooner under the command of Arthur Gallant who himself received
Oshkosh Grand Champion for his Lancair 320. I learned a lot on that project. The following
describes in great details the techniques I use to obtain a perfect straight wave free airfoil shape
on wings and the fuselage, and do it with the minimum effort and cost without spending years on
it (or so I think).

Pinholes
These are tiny voids, bubbles and pits caused by air mixed into the filler. They are invisible until
the minute you start spraying the first coat of primer. They can give you a big headache if you
attack them the wrong way. You might have heard horror stories of builders spraying coat after
coat of primer trying to get rid of them. The bottom line is, you cannot fill the pinholes by
spraying. Yes, if you keep spraying long enough, eventually they will disappear, but they will
not be filled, they will be bridged. Bridged pinholes can cause the painted surface to develop
little pimple like bumps on the sun when the paint softens some and the trapped air expands with
the heat.
Luckily, we have a simple effective way how to deal with pinholes before we even see any.
Vacuum the surface real well to remove any dust and then squeegee pure epoxy resin over the
surface. The coat is very thin and the amount of resin is mall, you are basically just wetting the
surface, give it some time to soak in and squeegee off all the excess. The resin has very low
surface tension so it flows into all those small voids and because unlike primers it doesn’t
contain any volatiles it doesn’t shrink as it cures so the fill is complete. The second benefit of
this step is that the resins hardens the top shell of the micro, making it more durable.
A word of caution, many epoxy resins do not cure well and stay gummy at very thin coat,
especially in humid condition. If yours is one of those or you are not sure, use the West system
epoxy for this.
When this top coat is cured sand it lightly with 100 just to break the gloss, and you are ready for
the primer.