Robert

Jim Weir tends at times to be a little less that patient, at other
times tends to disseminate information that is less than correct, and
at times tends to operate in the margins of legality, but his
credentials here are not in question. And you are being just a little
bit paranoid.

A Form 337 is an FAA record of major repair or alteration. The ONLY
time one is required is when you perform a major repair or alteration.
A major alteration requires approved data, and must be signed by an IA.
The sticking point is what constitutes approved data. An STC is
approved data. So is an FAA field approval (which you won't get), or a
DER's report. Everything else is iffy. I've had 337's bounced back
because an FAA-approved (complete with serial number, revision date,
and revision history) installation manual for a TSO'd component was
judged not to be approved data. Two years prior, the exact same manual
was considered by the same FSDO to be approved data and the 337 was
accepted.

What I am trying to say, ever so gently, is that there is no such thing
as a cost-effective clear-cut answer when it comes to altering an
aircraft. If you insist on getting a clear-cut, black-and-white, no
shades of gray kind of answer, you will be going down the road of
needing a DER to generate approved data for your major alteration
requiring a 337. That's not reasonable, but that's the CYA answer
you're going to get. It's the CYA answer the dealer gave you. He
didn't want to jump through the hoops of getting an STC or PMA, but if
he even suggests that you install his plastic without approved data,
all it takes is some fed to decide it's major and he's looking at a rap
for selling unapproved parts.

Another way to look at this (the way you're trying to do it) is that
you are installing a decorative or preservative coating which is not
prohibited and not contrary to good aviation practice. That's
preventive maintenance, and requires nothing more than a logbook entry
made by you. So who makes the determination that what you are doing is
preventive maintenance rather than an alteration? Well, you do. And
your determination stands until some fed (or overly officious IA doing
your annual) decides otherwise. Of course you could call your FSDO and
ask for an official determination (in writing). The determination you
will probably get (since it's the safest one for the fed to issue) is
that unless you show otherwise, you should hire a DER and get an STC.
The fed is not likely to stick his neck out and say it's minor - that
exposes him to risk, and there's nothing in it for him.

This is not a good way to do things, but it's what we are stuck with.

One of the reasons I maintain a certain amount of anonymity on the net
(even though anyone with half a brain and minimal skills in using
Google can figure out - but not legally prove - exactly who I am) is so
I can give out advice that could get my A&P revoked. Here's my advice.
Put in the plastic. Don't sign a damn thing. It was always there.

Or you can make the logbook entry. It will probably be OK - but it can
be used to violate you.

Michael