Gelcoat Sanding

[Peter von Tresckow]

So after attending Uncle Hank's presentation at the convention I have been
inspired to sand out the crazing in my gel coat. I am planning on the
600-1000-1200-1500 wet sanding process and then buffing out the finish.

I understand the basics, but does anyone have a good primer on the process?

Thanks

Peter

[ND]

hi Peter, here's the finish sanding process I use on every glider we refinish. first let me say there are many ways to arrive at the same result. i'm the blonde guy hank called on near the end who was standing against the side wall.

i take sections of wing and draw a line with pencil from leading edge to trailing edge every 18 inches or so. then, with 800 (you can use 600) you'll sand two sections at a time using spanwise strokes, working from leading edge to trailing edge. each time you finish two sections, move up 1 section, so you are overlapping and hitting each section two times as you move down the wing. repeat the process for each grit. truth be told, i do 800, 1000, and 1500. change paper alot, it goes dead quickly. especially with old hard gel-coat.

now, for the literal coup de grâce: after doing sections of 1500, wet the whole wing down, and walk back and forth, root to tip, doing single strokes that last the whole wingspan using spent 1500. you won't be sorry. if you do a ton of that at the end, the wing will look buffed before it's buffed, and it will also shine up much easier.

if you want more detailed explanation on the actual process, email me: brayera at gmail dot com and i'll give you my number so we can talk. if your wing is crazed, don't wait to do this. the longer you wait, the worse off you are. sometimes, if you catch crazing **early**, you can save the finish. no matter what stage your crazing is at, polish hard and often from now on and you'll squeeze some extra years out of your finish.

-Andy

[Michael Opitz]

>i take sections of wing and draw a line with pencil from leading edge
to

>tr=

>ailing edge every 18 inches or so. then, with 800 (you can use 600)
you'll

>=

>sand two sections at a time using spanwise strokes, working from
leading

>ed=
>ge to trailing edge.

Did you really mean spanwise and not chordwise??? When I worked at
S-H, we NEVER sanded spanwise. It was either chordwise or
alternating 45 degrees off chordwise. FWIW......

[Charlie M. (UH & 002 owner/pilot)]

On Thursday, March 24, 2016 at 4:20:54 PM UTC-4, vontresc wrote:

ND gave a decent reply.

If hand sanding, start saying to yourself, "I must, I must, I must build up my bust....".
I like to say, "Sanding Gelcoat is like polishing granite with wet toilet paper".

Lot's of work.

A decent random orbital sander with disc's (going from 600 wet to 3000 wet) can make a HUGE difference. Using a block or spline board will work, but from my view, it's a LOT more work.
There's a reason a decent shop charges what they do, "they've been there, done that".

Final polish/wax also depends on the tools/wax you use.
So states the "polish guy" (job, not country..) on a number of projects.

[Someone in the shop today called me, "Buffy the dull slayer".... sorta fits.]

[2G]

How do you sand the leading edge?

[Mark628CA]

So far, nobody has mentioned supporting the wing in at least three places to make sure the surface is straight spanwise. Just an inch or so of sag and your hard work will result in very questionable results as your two or three foot long sanding block exerts pressure on its ends and the center bends according to the weight on your hands and arms. You can bet that the curve (and rebound as you push and release) will make your efforts totally random if you are trying for an accurate, straight surface.

Learned this from Fidel Ramirez, a guy who has spent thirty-five years becoming a Master at 'glass repair and shiny white things that fly.

[Mark628CA]

Another hint: Add a bit of dishwashing soap to the water you use to clean off your wet sanding paper and rinse the wing surface. It reduces the amount of residue that builds up on the sandpaper and makes it last longer.

And remember: Patience is a virtue. Take your time. And then take some more time. Good things come to those who exhibit patience.

At least with things like this. Other things require more immediate results. That's why high explosives were invented. Mining would take a LONG time if you had to use sandpaper.

[uncl...@ix.netcom.com]

On Thursday, March 24, 2016 at 4:20:54 PM UTC-4, vontresc wrote:

Start with one grit finer than you think you need. Finer grit leaves less scratching to remove later.
If you guessed wrong it will take a lot of time to flatten the crazing. If so go down one step. I never go below 600 as it takes forever to get out scratches.
Except for edges and ends, we do it all with the orbital.
My sanding pattern is(remember my example where I dropped the mike?) is left to right as far as I can hold a consistent pattern- about 24 inches or so. 2 traverses at the start, then shift about 1/2 inch on each pass. When all the way across, do the same in reverse. Finish off with one set of passes at 90 degrees with the same offset. Keep the surface wet.
Note where the end of your pattern is and set up again with enough offset to get uniform coverage.
A 1000 grit disc will go about 4 feet on a Std class wing and then not much is happening.
I block and finger sand leading edges and tips.
When crazing is flatted, do the same with 1500 on orbital.
Then 3000.
Buff.
Wear wet weather shoes.
UH

[Charlie M. (UH & 002 owner/pilot)]

I will a little bit to here......

"It all depends on the sludge". So, what do I mean?

Depends on the grit you're using and how do you clean the surface.

I like a "fairly cheap ~12" red rubber squeegee", (the black ones tend to degrade and leave dark schmutch [sp] on the surface....) to remove water and "stuff" from the surface.
When using coarse grades of paper, a "good paper" will make a lot of "semi-solid" stuff come off with the water when new, not so much when worn.
When you get to fine paper/pads, even new will produce less "semi-solid" stuff with the water than coarse grit worn.
When either produces a lot less "semi-solid" stuff compared to new paper/pads (of a certain grade/grit), it may be time to change the paper/pad.

So, starting at 600grit wet, fresh paper yields a "sludge" of near "wet yogurt" when new, going to "2% milk" when worn.
Starting at 3000 grit wet, fresh paper/pad yields "whole milk" when new going to "diluted skim milk" when worn.

This sorta takes into consideration of different area volumes you may cover, how wet or dry the surface is, how hard you press, etc.
It's all in the sludge.

The better the sanding, the better the polish can be. It's always faster to do a bit more sanding than to try & polish up a poor sanding job.

[Coarse in this post is 600 grit wet, fine/final is 3000 grit wet with several steps in between]

[Casey]

I've seen several different grits of compound take out sanding marks, But yes, fine sanding marks makes a better finish with less polishing. But polishing takes off less material.

[busha...@gmail.com]

Rather than pencil marks I recommend 3M dry guide coat. Even when wet sanding.

Lane

[ND]

i really mean spanwise, when i worked at schempp-hirth 6 years ago they always sanded spanwise for finish sanding. and i have been doing it that way at a nationally known glider refinishing company for 12 years.

[ND]

for finish sanding im not recommending that, because you are using a soft block for spanwise sanding. if you are using a hardblock for orange peel removal, or heavier gelcoat removal, you would want to support the wing in three places, or whatever it takes to keep the wing from sagging. on an LS-8, you only need two stands because they're so still they don't sag.

[ND]

On Thursday, March 24, 2016 at 9:49:47 PM UTC-4, Mark628CA wrote:
but soapy residue also corrodes metal faster. so if oyu use any soap (we never do) make sure you take steps to protect your contol linkages, and hanges.

[ND]

guide coat is good for taking away orange peel or a spar bump, but pencil makes a much smaller mess -AND- if he uses guide coat, he'll have black lines on his wing forever, since it will wick down into any remaining crazing (unless he plans on sanding the crazing away entirely, but that may also mean sanding the gelcoat away entirely, depending on how severe it is).

The technique i am describing is only finish sanding. the pencil is there to show what section to sand in, and the chord-wise pencil line lets you know if you've removed enough material/ not missed spots as you sand spannwise.

-ND

[ND]

also, for the process i describe, you'll want to use a soft finish sanding block. like the one in the attached link:

http://homefixated.flashpointstudio.netdna-cdn.com/wp-content/uploads/2010/05/Festool-Sanding-Block-Round.jpg

[Ramy]

I often hear that sanding is only a temporary solution, and after couple of years the gel coat will lift again and now you have a thiner gel coat layer and wider cracks. If so, why sanding unless you try to sell your glider?

Ramy

[Charlie M. (UH & 002 owner/pilot)]

On Monday, March 28, 2016 at 12:08:22 PM UTC-4, Ramy wrote:
> I often hear that sanding is only a temporary solution, and after couple of years the gel coat will lift again and now you have a thinner gel coat layer and wider cracks. If so, why sanding unless you try to sell your glider?
>
> Ramy

So you can save up for a full refinish. A "flattening sanding" helps with the aerodynamics followed by a good wax/polish can extend the life of what you have. A full refinish is not cheap.

[ND]

Hi Ramy,

i recently read an article on you in gliding international. you are an inspiration, i can only imagines some of the beautiful scenery you must have seen!

another possible reason to sand is if you can catch crazing in it's very earliest stages you can sand away the damaged gelcoat and literally remove the crazing completely. crazing most typically begins in the existing sanding scratches on the surface of the gelcoat. that's the reason they are often chordwise, because as mike opitz indicated above, the manufacturers used to sand chordwise.

On Monday, March 28, 2016 at 12:08:22 PM UTC-4, Ramy wrote:

[Papa3]

A friend of mine in the auto body business turned me on to 3M Dry Guide Coat, and I'm a huge fan. Used to use leftover rattle can paint and/or pencil, but this stuff is much better. It goes on dry, gets down into the pinholes and crannies, and doesn't clog the paper when you sand (if anything, I think it's a lubricant). Plus, you can get it out with some mild solvent (like Prep Kleen). Note: That's for wet-sanding new paint to get rid of the orange peel or identify low spots etc. What Andy is talking about is different, since it's failed/failing gelcoat. FWIW.

[ND]

agreed! i love guide coat too. it shows EVERYTHING, even bad sanding technique, or a dinged up block.

[2G]

Check this out (I also posted on the Trizact thread):

https://www.youtube.com/watch?v=ke5kfYz9Ybw

[uncl...@ix.netcom.com]

Note the sanding pattern demonstrated which is like what I was trying to describe above.
We use the 3M compounds and polishes for paint but use edge buffer applied hard wax/rouge for final buff and to impregnate/seal the gelcoat which has a lot of porosity.
UH