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I think I have denuded enough sandpaper over the years to add
my two cents worth to the discussion on priming/prepping composite surfaces for
final paint. First, no matter what primers are eventually used I think
it's extremely important to properly prep the surfaces to be painted. By
the time paint is applied these parts have had plenty of opportunity to become
contaminated with mold release, finger print oils, hydraulic fluid residue, beer
foam, and whatever other aerosols that float around the shop. I sand the
surfaces with 80 grit and then wash them very thoroughly with prep-sol, acetone,
lacquer thinner, and/or whatever good solvent I have. At the
suggestion of the person who was to do the final painting we started using PPG
K-38 high build primer surfacer as the initial covering. This is applied
with a 4" paint roller, unthinned, and does a great job of filling any pinholes,
scratches, bare micro, etc. It tacks up quick enough that a couple coats
can be applied in quick succession, giving a good build up. This is
sandable in an hour or so. All the sanding at this point is done with 80
grit dry. If you use a long board on the flat surfaces, like wings and
tail, the high and low spots show up well and will probably level out with a
couple applications. Then, again per the painter, a coat of PPG K-36
primer is sprayed on. If spraying is impractical this can also be rolled
on and lightly sanded to remove roller marks.
I really don't like the idea of flying in primer for many
reasons but it's often the only choice. Primers don't provide a good
smooth surface and tend to act like a sponge to exhaust residue and other
contaminants. A professional painter, if he's good, will sand off most of
the primer/surfacer you've put on and clean everything real good and then start
over in the paint process. If not, it's likely fisheyes and other bad
things will happen.
I have used WLS primer and have no doubt that it does
produce a great end result. However, I don't like it's very slow curing
time. Like any epoxy product it feels cured but is not ready to sand for
many hours, preferably overnight. In talking to some experienced body shop
experts they have told me that on bare metal the epoxy primers are definitely
the best choice for their superior adhesion but on composite surfaces they think
it's overkill and just as good a result can be had with the urethane
primer/surfacers. One of these professionals works almost exclusively with
aftermarket fiberglass street rod bodies and I tend to believe what he
says.
Recently I built an epoxy/fiberglass replica sports car body
from scratch (if I start another one will someone please shoot me) and it
required a lot more surface finishing that your average Lancair. In
shopping for a more economical primer/surfacer I discovered a product at my
local NAPA (National Airplane Parts Association) store that is about a third
less expensive than the PPG products. It's their house brand (made by
Martin Senour) called Crossfire 2K Fast Filling HB Primer, number 15222.
It's a two part urethane and after using a couple gallons, so far, on the car
body I like it very much. Covers good, dries quick, and sands well.
And it's cheaper.
These are just some observations and opinions I have formed in
the process of building five Lancairs and a few other toys and should not be
taken as more than just that; my opinion. The reason we put the word
"experimental" on our planes is so we can do them the way we want, within
reason. I'm sure everyone that has prepped a plane for paint has his/her
own ideas on the best way to do it and whatever works for the individual is what
they should do. The main purpose of the LML, in my opinion, is to share
what has or has not worked and let each builder wade through the info and reach
their own conclusions.
Gotta go, I hear the sandpaper calling.
Leighton Mangels
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