Fear of Sanding

[Stuart Grant]

Wings of my 87 Discus had a light craze when I got it two years ago.
About a year ago people who had owned glass gliders a long time said I
really should sand it. Crazing got worse this past year so I spent
almost a week of my vacation between Christmas and the New Year sanding
my wings. With a little guidance from friends it was really pretty easy.
Started with 400 then 600, 800, 1200, 1500 and finished with 3M Finesse.
I will be doing the wax (WX-Block/Shield) this weekend. Did the coarser
sanding at 45 degree angles to the cord with a 10 inch "preppin-weapon"
sanding block and the 1200 and 1500 as full sheets by hand at 90 degrees
and parallel. All was done wet. Used about 8 sheets each of 400, 600 and
800 in quarters and 4 each of the 1200 and 1500. 1 full bottle of
Finesse applied with a 7" polisher and the edge of cloth wheels. Wasn't
strenuous work just dish-pan hands form being wet and sore feet from
standing. I think you can do a wing in about 12 hours once you get the
hang of it. Gel coat is remarkably easy to sand. I did a patch about the
size of my palm that came out just fine other than color difference. It
would be nice if someone who has done it more than a few times would
write an article on the fine points for Soaring Magazine. Bottom line is
that sanding and small gel coat repairs are relatively easy, require
minimal materials and equipment and no extraordinary skill except for
color match. Have no fear.

[Robert Ehrlich]

Stuart Grant wrote:
>
> Wings of my 87 Discus had a light craze when I got it two years ago.
> About a year ago people who had owned glass gliders a long time said I
> really should sand it. Crazing got worse this past year so I spent
> almost a week of my vacation between Christmas and the New Year sanding
> my wings. With a little guidance from friends it was really pretty easy.
> Started with 400 then 600, 800, 1200, 1500 and finished with 3M Finesse.
> I will be doing the wax (WX-Block/Shield) this weekend. Did the coarser
> sanding at 45 degree angles to the cord with a 10 inch "preppin-weapon"
> sanding block and the 1200 and 1500 as full sheets by hand at 90 degrees
> and parallel. All was done wet. Used about 8 sheets each of 400, 600 and
> 800 in quarters and 4 each of the 1200 and 1500. 1 full bottle of
> Finesse applied with a 7" polisher and the edge of cloth wheels. Wasn't
> strenuous work just dish-pan hands form being wet and sore feet from
> standing. I think you can do a wing in about 12 hours once you get the
> hang of it. Gel coat is remarkably easy to sand. I did a patch about the
> size of my palm that came out just fine other than color difference. It
> would be nice if someone who has done it more than a few times would
> write an article on the fine points for Soaring Magazine. Bottom line is
> that sanding and small gel coat repairs are relatively easy, require
> minimal materials and equipment and no extraordinary skill except for
> color match. Have no fear.


This is done almost routinely in my club. The club has a permanent employee
who manages the flights during the soaring season and do the maintenance
work during winter. As part of this job he repairs the various scratches
on bellies or wing following outlandings in the way you describe and if
times remaining permits (we have 20 "plastic" gliders) he completely
refinishes the glider with the most cracked gelcoat (the last time this happened
was 2 years ago). Under his supervision a private owner also completely refinished
the fuselage of his Nimbus3 last winter.

When I watched this work, I saw they used a trick that you didn't mention.
Before each wet sanding pass, they paint the surface to sand with red paint
very highly diluted with aceton. This helps to stop the sanding where/when
the paint is gone by sanding.

[[email protected]]

Robert,

I think the paint trick is a good idea. After my final buffing I can
see that I missed some areas with the finer grades of sandpaper -
leaving some scratches visible in the right lighting when viewed from a
low angle. You can't see these until buffing is almost complete. Using
weak paint before each grade of sandpaper would avoid this problem.
Will try it next time!

Stuart


Robert Ehrlich wrote:
> This is done almost routinely in my club. The club has a permanent
employee
> who manages the flights during the soaring season and do the
maintenance
> work during winter. As part of this job he repairs the various
scratches
> on bellies or wing following outlandings in the way you describe and
if
> times remaining permits (we have 20 "plastic" gliders) he completely
> refinishes the glider with the most cracked gelcoat (the last time
this happened
> was 2 years ago). Under his supervision a private owner also
completely refinished
> the fuselage of his Nimbus3 last winter.
>
> When I watched this work, I saw they used a trick that you didn't
mention.
> Before each wet sanding pass, they paint the surface to sand with red
paint
> very highly diluted with aceton. This helps to stop the sanding
where/when
> the paint is gone by sanding.

[Roy Bourgeois]

A similar technique is used to deal with the "orange peel" that develops
when first spraying Prestec. See Kochanski's article at the SRA site. He
uses thinned blue dope. Also helpful: I use large carpenter's lead pencil
to mark up the border area between old and new gel coat. When the pencil
mark disappears - it's time to STOP sanding.

Roy A. Bourgeois

[[email protected]]

The pencil technique works well: just scribble all over the area you're
sanding and stop when the pencil marks disappear. Or, if they're in a
small depression and the area around it is white, wipe off the pencil
marks and fill.

Regarding scratches, a repair expert showed me one way to make sure the
scratches are gone before moving on: sand only in one 45-degree
direction per grade of sandpaper...and then switch directions when
moving up to the next finer grade. So if you're sanding with 600 grit
in an upper-right-to-lower-left direction, don't stop until all the
[400 grit] sanding marks/scratches in the upper-left-to-lower-right
direction are gone.

He also demonstrated how I'd been using sandpaper far too long. 400
grit cuts great, for example, when it's new, but I'd been trying to
conserve paper and working WAY too hard (and long) to get results long
after the paper wasn't cutting much anymore.

There are lots of little tricks and hints to wing sanding. Ken
Kochanski's article is helpful. It would be great if someone pulled
together the collected wisdom of repair experts and inveterate tuners
for the rest of us.

Chip Bearden
ASW-24 "JB"

Roy Bourgeois wrote:
> A similar technique is used to deal with the "orange peel" that
develops
> when first spraying Prestec. See Kochanski's article at the SRA
site. He
> uses thinned blue dope. Also helpful: I use large carpenter's lead
pencil
> to mark up the border area between old and new gel coat. When the
pencil
> mark disappears - it's time to STOP sanding.
>
> Roy A. Bourgeois

[Papa3]

I learned another neat trick a while back from an auto-body guy related
to Chip's comment. This applies if you're repairing or sanding a
limited area (e.g. just the outboard 3 feet of wing). A common
problem is "managing" the size of the patch or sanded area and
especially the feathering to the untouched areas. ie. Since you
always want to make sure you remove the coarser grit scratches, you
need to make sure that each finer grit completely covers the area
covered by the prior grit. In addition to changing the bias on the 45
degree passes, you can manage the edges/feathering of the patch by
using successive layers of masking tape. You apply the tape in
"reverse order"; ie. you put the outside layer on first. Then, you
slightly overlap successive layers, working in toward the area you want
to contain. You need as many layers as grits.

So, if you are going to be doing 5 passes (400, 600, 800, 1000, 1200),
you need 5 layers of tape. After you do the 400 pass, you peel away
the inner-most row of tape, thus exposing another inch or so of
"interface area". When you do the 600 grit, this ensures that you
cover the entire 400 grit area plus "a little bit." Same with the
800, etc. (in reality, this is most critical for the 400, 600, and
800 grits; once you get beyond that, it's pretty painless to manage )
It's easier to do than it is to describe.

[Nolaminar]

For final sanding, it is good to use a pencil. You can go wild with lines and
circles with soft lead pencil and it will not clog fine sandpaper.
GA

[For Example John Smith]

Thanks to all for the posts in this thread. My partner and started a
sanding project on our 1977 Glasflugel Mosquito on Saturday. The wings and
tail were recently refinished but not the fuselage. We thought we'd need to
get it redone as well.
After overcoming our fear of sanding, we think we can get a few more years
out of the fuselage before refinishing.

Brent
"Nolaminar" > wrote in message
...
> For final sanding, it is good to use a pencil. You can go wild with lines
and
> circles with soft lead pencil and it will not clog fine sandpaper.
> GA
>