On Tue, 17 Jun 1997 PARADOX@DEPAUW.EDU wrote:
> A friend is restoring a Cessna 172 airplane, and decided to do his own
> painting. I helped in matters of advising re: equipment and interpreting
> instructions, and so on. He's pretty well satisfied with the results
> (using Martin Senour automotive acrylic enamel) so far, but there are three
> questions that mightier minds than mine could help out on:
>
> 1) When the instructions say "thin" coats, what do they mean? After one
> thin coat on bare aluminum, should the aluminum still shine through, or is
> that too thin?
Probably not too thin; it is unusual to cover completely in one coat.
But no primer???
>
> 2) He has used the proper amount of the proper reducer, but no hardener
> mixed into the paint. What would be (have been) the advantages of doing
> so?
He could have rubbed out the roughness you describe later, if it was not
too bad.
Of using a top coat of hardener?
I'm not sure you can use hardener in only the top coat.
[A subquestion, out of curiosity:
> Reading between the lines of the instructions on a can of laquer thinner he
> bought for cleaning the equipment, one gets the following impression - that
> laquer thinner is meant for thinning laquer (reasonably enough) and that
> other paints have other reducers, and you should use them, but if you did
> use this laquer thinner for thinning them, it would be like a medium speed
> reducer made for middle range temperatures. Could he really have saved $15
> a gallon by going this route?]
Lacquer thinner can be used to thin enamel, at least I've done it, but it
dries too fast and heaven knows how it will affect what you are painting
over (lacquer thinner will lift fresh enamel). Not recommended.
> 3) The big one - the first coat, conscientiously applied, dries to a high
> sheen (but, if thin, isn't enough coverage). Subsequent coats suffer from
> a dulling - it appears at the edges of the path of the spray and looks like
> paint that was too dry when it contacted the surface. (The first coat does
> this as well, but subsequent passes add to this "overspray" and it
> liquifies and forms a solid coat.
It does not really liquify, but it gets covered up.
Second coats don't seem to work this
> way.) When we did the vertical stabilizer, this wasn't a problem since the
> area to be painted was relatively small. But on the wings, these dull area
> never could be painted out, presumably because of the drying that takes
> place before you get back to the place you started. If he paints parallel
> to the leading edge of the wing, The result is zebra-like: alternating
> lines of shiny and dull paint. I suspect that rubbing compound might be of
> benefit, but he doesn't want to hear that. Would different thinning on
> second coats improve things? Or changes in other parameters?
The paint at the edges of the fan is drying before it has a chance to flow
out. To minimize the problem, he should paint so as to keep a wet edge.
It might have been best to put the first coat on parallel to the leading
edge, and later coats perpendicular to the leading edge. The next thing
that would help is to use a slower drying reducer. Finally, I suspect he
is not overlapping his strokes as much as he should. If you overlap them,
the edge of stroke 2 falls near the center of stroke 1, so the overspray
at the edges of stroke 2 falls in the wet paint in the center of the first
stroke. Finally, increasing the amount of thinner a bit with each coat
can help. I guess it goes without saying that you don't paint in the
sun... or does it?
W. R. Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8629
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