Saturday, February 28, 2009


Sanding gives you some time to think. So I was excited to start fresh on the second side. I wanted to try a couple of new ideas on top of what I'd already learned. My dream was to get all the QF on and then sand it off, all in two sessions (had I gone from depression to

One of my first ideas was to fine-tune the guides. Several issues presented themselves.

1) I felt I was learning what the long board was capable of. 

It's a great 'averager' of heights. While it can do some amazing things, if you give it a variety of heights (or in the case of stripes, even a variety of widths) you will get an average that is some place lower than the lowest height. (Or, "if you give it junk, you get back below-average junk.")

2) Getting out the straight edge is a necessity.

The first time I sanded down the stripes, I sanded till the long board touched everywhere. What became apparent later was that sanding till it looks sanded isn't a very good endpoint. Many parts of the float are nearly flat fore and aft, and you can have a long 'wave' of high and low develop. And, with enough force it seems that the flexible board can dish-out areas (especially if you get confused and start sanding low spots, versus sand the high spots like you should). The straight edge doesn't lie.

I got the straight edge out near the end of the first side, but my thought for this side was to use it on the guides at the beginning.

You can sight along the straight edge (SE), but better yet, listen. After a little sanding, you can rock the SE along the stripes from deck to keel and get a smooth, non-clicking motion. This is the easiest dimension to achieve. Do this fore and aft, and at first you will get a non-uniform motion with one or more slapping sounds. Keep sanding and an eventual fair shape will produce an even motion with an even clicking sound as you rock over each stripe in turn. A slap indicates that the SE is rotating over a high spot or bridging a low spot.

Then I noticed that some graphite from previous markings along the SE was being left behind. I added more along the edge and marked the stripes by using a rocking motion starting at the deck and working down to the keel. The whole side was finished in a couple minutes.

Marks on all adjacent stripes means fair.

A stripe (or two) without marks means it's too low, or one of the adjacent sides are too high.

Sighting along the SE at these locations usually suggested the appropriate action and I developed an easy marking system so I could go along and spot-sand, fixing the problems. With minimal effort I now felt that I was starting with guides that were intentionally high, but fair.

I filled between the stripes as before. I think I forgot to mention that I would sand the edges of the stripes. Not really a fun way to spend an hour, but how could you leave a surface that you know will not bond secondarily?

In the picture below you can see a problem, similar to the other side, of a low area just above the keel. I could see this while I was planking, but any maneuver to fix it seemed to cause more troubles elsewhere, so here I am. Also (as seen in pictures further down), I discovered that feathering the stripes at the deck and keel like this before filling was a mistake. Leave them high until the end.

You can see in the last picture how I marked with pencil along each stripe. After filling, there will be some high ridges that are easy to sand away, but I was concerned about removing as much of the junk as possible before I actually started fairing. My plan was to spot-sand down to the marked surface to increase my odds of starting fair.

You can see that after the first couple passes I need to correct some areas to be at the 'fair guide' height. You can also see why high guides are important. And you can see what is developing at the keel by feathering the stripes before filling. Too much material is getting removed too quickly.

I'm already concerned that I didn't leave the guides high enough. Only moderate sanding and I'm starting to mark spots where I'm concerned that I removed too much material. The glass showing along the keel is the edge that was wrapped around from the other side, so as it shows I'm really just feathering its edge. You can also see how just above the keel, feathering the stripes too low (to clarify, sanded to the correct final height before filling) has caused a row of low spots.

At some point I decided to stop and fill. The rows of little dabs of QF are filling bubbles that were in the stripes. Obviously they showed up during mixing and came out the Zip-lock candy bag. I enlarged these with the Dremel as before.

Below shows an idea for trying to fill and spot-sand a low spot. The goal is to minimize its effect on lowering the surrounding finished height during final sanding. (I find sanding addicting -- a little more . . . Now, before filling I again find myself almost, if not at, the stopping height.) I marked the surface with diagonal marks, sanded for adhesion, then marked the perimeter of the sanded area with a irregular circle. After filling a thin, but adequate amount, I removed the excess outside the circle with a putty knife.

Then I attempted to spot-sand, using the usual motions, until I was just sanding on the surrounding glossy surface left by the putty knife. I also spot-sanded the dabs filling the bubbles.

I think the above ideas were reasonable, but suffice it to say that all my efforts to do it in one big session -- or now, even one spot fill -- weren't going to work. I was forced to stop before I sanded on glass.

What I discoved (not a new discovery . . . sometimes we just have to experience things for ourselves) is that when you first sand through the fairing compound, it's tempting to continue on the epoxy. This hard spot can remain for quite a few passes depending on pressure and grit before any damage is done to the glass. You're thinking, "I'm not on the glass yet!" But of course you've disrupted your fair surface by adding in a new substrate with different properties. The surrounding QF continues to lower at a faster rate than the epoxy. The straight edge never lies. The only way I felt I could take care of little problems here and there was to skim coat again.

Here it is after a couple passes. What you see is a pencil mark through the skim coat, marking an area of concern, i.e. the top of a high spot. As the QF thins it becomes transparent and is a great help in knowing when the stopping point is getting close.

When it was close I changed to 60 grit and called it good. Not quite the one on, one off I had been hoping, but I was happy with the end result.

At this point, I had a major breakthrough.

I flipped the hull upside-down to fine-tune the keel. It seemed I had a couple of low spots, so I filled these, and re-sanded. This time it looked like I had different low spots. I know that one thing affects another, but this seemed ridiculous.

The day before I had purchased a RIGID 3M Hookit longboard, thinking that it would be useful for the flat surface of the deck. I gingerly tried using it along the keel and immediately felt like it might be giving me 'truer' results than the flexible.

Before going too far I stopped to do some research, wondering about this whole flexible versus rigid thing. I had started thinking that curved=flexible and flat=rigid. I came upon a website from the experimental airplane building community, part of Wayne Hicks' Cozy IV project. I read 'Chapter 25: The Contouring Process' several times. Part of me was sad because I had already spent so much time learning some of these things for myself (at least my thoughts weren't far off); the other part of me was excited to try all the new ideas.
I sent Wayne an email, and he responded in a couple of hours.

Wayne, enjoyed your site.
---> Thanx!

---> Cool! I've heard of 'em. I used to race Tornado catamarans.

My question was about using the flexible vs. rigid long board (3M Hookit). I have been exclusively using the flexible for the long convex shape of the outside of the hulls, but recently purchased the rigid as the deck on the floats is flat. Does the rigid board have any role for this curved surface?
----> I have found that the flat board works better than the flexi board when contouring large-area curved surfaces. Here's why - the flexi board is TOO flexible. The rigid board allows for better control. This is especially true if you overlap your strokes by an inch or two. They say the human eye can only detect imperfections in contour to about 3 feet. Even most large-area surface contours are effectively "flat" at that length.

I can't overstate how switching to the rigid long board improved my end result. I would also say that reading 'Chapter 25' is a must for all builders embarking on the fairing challenge.

The first new idea I wanted to try was the "skim coat before raw epoxy finishing technique" for scratches and pin holes (see step 5 in chapter 25). After all the effort of fairing the sides, I was anxious about slathering on some mixture with a squeegee, so decided to try it on the deck UNDER the QF. I had a couple of areas that I could experiment on, that had something similar to pin holes where the area between the glass tows were light on epoxy (and I had previously found that QF was difficult to get into very small imperfections). I did it just like he described, and it gave me enough confidence to plan on doing the whole hull. Here is a close up.

Before it cured I did a skim coat of QF over the deck and transom. I'm still sold on the candy-stripe method for curved surfaces, but I know what Menno was saying about the extra work, so tried without.

It was four days before I got back to it. QF gets harder! Still, the end result worked out well. I played with what direction to work the long-board. The usual orientation -- keeping the long-board pointing towards bow/stern -- could leave a camber in the deck. I countered this by also adding a bow-to-stern motion with the board oriented at 45°, then back the other 45°. The skim coat method worked well, given that I was sure the deck was flat by rebating the foam in areas with two layers of glass and having it flat against the strong back while attaching it.

QuickFair requires several coats of epoxy sealer. Originally I figured that any scratches from sanding would be taken care of by these layers. After reading Wayne's advice, I modified this to include a skim coat of low-density filler with a soft, creamy consistency. I applied this with a squeegee, just as I had experimented with on the deck.

I worked it into pin holes etc., smoothed uniformly, then removed with a forceful drag of the squeegee. What I removed was pushed forward to a new area. I actually used very little material. I did discover that the QF (I assume) slowly absorbed the epoxy, causing the mixture to become thicker. So after moving it forward several sections, I could discard it for new mixture. Using a resin mixture with a long work time was so pleasant after the 10 minute work time of QF.

What a great idea! I was pleased with the results. It seems that my 36 grit has some clumps of abrasive which I'll try to watch for in the future, but I think this technique is a great way to take care of these and other small imperfections.

When this got tacky, I rolled and tipped a layer of epoxy. Seeing that first, shiny coat was pretty fun. When cured enough I put on a second layer. After this layer, all the imperfections were hidden.

Off to storage, switching places with the other float.

The second float seemed so far behind. Luckily I had done all the interior work. Having done this once before, I was able to be more efficient with certain steps. Here I filled screw holes.

Below: fitting my three deck pieces together and cutting out the deck flange for the stiffeners. (I joined the pieces while the hull was upright, but I did the interior taping while the deck was flat on the strong back, just before joining to the hull. I had previously rebated the pieces so as to not make high spots on the finished deck.)

Between steps -- while things cured -- I began to think about other small parts. Below is the mold for the rudder gudgeons.

Since I made two foam bows earlier, it was a simple matter to attach the second before the deck bond had even cured.


Jay said...

Wow - this is a really great informational post Andrew! Thanks for the detailed writeup. (Need to see more of these from you, hint hint :)).


Tor Rabe said...

Great post, Andrew. And a really great looking float as well. I'm almost looking forward to the main hull fairing after reading all this, he, he

Andrew said...

Thanks for the nice comments. I've obviously been an ardent follower and learner from your blogs. Jay -looks like your house is coming along. Tor - congrats on the new arrival. Andrew.