Sanding on the chines

The manual and every guideline I've read warns against using a power sander on chines and edges because of the chance of cutting through the fiberglass.

 Well I did use a random orbital sander on my WD12, chines and all, and it does not appear that I have sanded into the fiberglass at any point.

Wouldn't it be obvious if I did? Wouldn't I see some fuzzies where the sander bit into the fiberglass?

I'm not disputing the wisdom of hand-sanding at such critical points. But since I've done the deed, I just want to determine whether I may have made an error that is invisible now but will show up later.

So what say you? Am I going to be sorry?


8 replies:

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RE: Sanding on the chines

Mike,

It's really obvious when you've sanded into the glass. You wouldn't see fuzzies, you'd see white areas thaht never turn clear, unless you've sanded all the way through, in which case you'll see wood.

CLC has to warn against it, otherwise they'd be answering tech support calls all day long from people who have sanded away their glass. But the truth is, oce abuilder has enough experience with a ROS and with glass, it's not going to be a problem. It's just a matter of a light touch. Just as in a recent thread there were a whole bunch of experienced builders fessing up to sanding with the edge of the disk, there's also a lot of us who power sand the chines with no trouble. Sounds like you're one of us.

 Laszlo

 

 

RE: Sanding on the chines

Laslo,  Here's another question !   I haven't got to the glassing stage yet, but have sanded the chines with a 6 inch ROS. I used it carefully, with a light touch (80 grit first, then a quick run with 120). 

All looks OK, but it is quite difficult to get a perfectly even round-over along plus-minus 16 feet of chine. So, when casting an eye down the length of the curve one notices slight imperfections (flat spots) i.e. the curve is slightly "out of fair".

I am now loth to put the ROS back on the boat. Would it help if I used a simple orbital-sander ?  Or better by hand, to even out the flat spots ?

Dave

RE: Sanding on the chines

Dave,

I'd definitely hand sand. In fact, sounds like what you need is a fairing board. In case you haven't used one before, it's about 2 - 3 feet long, 3 or 4 inches wide and has a vertical handle at each end. It's stiff but flexible (if you're making your own, 1/4 inch okoume works very well) with the sand paper on the bottom.

With a light to moderate pressure it rides the average shape of the hull, smoothing down the high spots without diving into the low.

You could run a fairing board fore-aft along the chines to clean out the wobbles, then touch it up with a sanding block. One important question is what kind of finsih are you planning. If it's paint, you might as well wait until the glass is on, fill the weave with fairing compound, put some extra along the chines and fair it then.

If it's varnish, then you need to be very careful not to take to top ply of the plywood off. It might be better to not get it completely fair before glassing and fair it after the weave is filled, instead (essentially using epoxy for your fairing compound).

Hope this helps,

Laszlo

 

RE: Sanding on the chines

Laszlo

Thanks very much for the extremely useful information.  Yes, I have read about fairing boards (I think in Wooden Boat Mag, which I subscribe to).  I shall make up a board and get on with it. There is only one short section that is bothering me.

As it happens, I will be painting the hull of my C16, so if all else fails I shall take your advice on using epoxy as a fairing compound.

D.

 

 

RE: Sanding on the chines

Dave,

You misunderstood. Only use epoxy as fairing compound if you're varnishing and need the transparency. If you're painting, use one of the compounds made by mixing epoxy and a filler (like phenolic microballoons).

Pure epoxy is too heavy and expensive to use under paint.

Laszlo

 

RE: Sanding on the chines

 

Oops !  Thanks for reminding me Laszlo.  I made up a board this-morning and used 120 grit on the curves.  Better, but still not there yet, However, rather than remove too much from the high-spots, I will fill.

I have a tin of West System 406 Colloidal Silica, this should do it.

One thing that does worry me about filling with an epoxy/microballoon mix are the never-ending warnings that John ((Harris) gives us about sanding epoxy. Having said this, I presume the "mix" will be easier to shape than "pure" epoxy.

I have never used microballoons.

Dave

RE: Sanding on the chines

Microballoons are very easy to sand since they're hollow plastic spheres. They are your best friend if you're painting. You can fill the weave with a slurry of microballoons and epoxy (instead of just epoxy), and you can fair with a stiffer mixture. The more microballoons you add, the lighter the resulting mix and the less epoxy you use. However, the stiffer mixtures tend to curl up onto the applicator. Making a less balloon rich mixture is one way to handle that. Another is to add a bit of cab-o-sil, but if you add too much the sanding becomes work again. Just experiment til you find the mix that is best for you.

Because the plastic spheres are extremely strong in compression, the fairing compound will resist bangs and knocks when cured. However, it is weak in shear and torsion, so it should never be used as a glue or as a structural member by itself.

When you sand the compound, the spheres break open and expose their hollows. Builders usuallly apply a final coat of epoxy to seal them, but this can get you into an infinite loop of sanding if there are drips. The answer is to either be very careful, or use a 2-part high build primer, such a s System 3's Yacht Primer, which is designed to fill in tiny holes.

The only real problem is that hideaous purple color, and there's nothing we can do about that until the primer covers it up.

Have fun,

Laszlo

 

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