Sunday, 26 May 2019

Sanding Fillets | This Vale of Tears

I have to admit that I became very despondent when faced with the task of sanding the fillets to a smooth curve. It was no surprise. I knew it was coming but I was still downhearted at the prospect of spending hours and hours scratching away at rock hard epoxy.

I made a half hearted attempt to begin sanding, using a piece of pipe insulation as a sanding block, and it barely scratched the surface. A crisis of motivation had set in.

I had read on one builder's blog that he was so demotivated by the prospect of sanding that he only managed one hour's work in the four weeks following completion of the fillets. I could easily empathise.

So I did nothing for a couple of days. While pondering how to find the energy to get going again it struck me that professional finishers must face this challenge every day, and they certainly don't do it by hand!

So I did some Googling and I discovered carbide burrs. A carbide burr is an industrial abrasive head mounted on an arbour for use in stationary or hand held grinding tools, for the purpose of deburring, shaping and finishing hard materials. A diamond cut burr will shape stainless steel and titanium, so it will handle epoxy with no difficulty.

I decided that a ball shaped burr would be best, and purchased one with a 16mm diameter - the largest I could find. Here it is.



A trial run with it fitted in my little DIY Bosch cordless drill worked quite well, but it quickly depleted the battery and the drill did not have the necessary grunt to drive the burr at the high speed which it needs to be effective.

My cordless professional  DeWalt drill is too big and heavy for use inside the boat, but luckily I had a third, small corded drill languishing in a drawer from many years ago. It is an ancient Bosch, so old it even has a chuck key!  Here it is, with the burr fitted.



It is powerful (400W) but is small enough and light enough to use inside tight spaces, and it proved ideal for dressing the fillets. It took four days to do the entire boat. The poor old Bosch got a bit tired after several hours of continuous use, but it held out.

I had removed the sharp and lumpy surface of the fillets, but they would still need to be sanded to a smooth finish - especially where they are visible. But I was happy to have otherwise saved a huge amount of work.

This is where I made my second discovery. I called the burr vendor to ask how long the burr will last, and in conversation he suggested that I consider SpiraBands. I had never heard of them, but I had a look and they seemed to be just what I needed to finish the fillets.

Again, they are industrial abrasives. They are cloth-backed sandpaper cylinders which slide onto a rubber holder mounted on an arbour for stationary or hand held use - ideal for use in an electric drill. They come in various lengths and diameters, although the choice of grits is quite limited - mostly coarse. I purchased a selection of sizes - 60, 30, 20, 15 and 10mm diameters. 

This is what they look like.



So then I went to work with the SpiraBands to smooth the fillets. Like the burr, they worked really well. I soon found that these abrasives work best on epoxy at low to medium speed, so I used my little variable speed Bosch cordless drill. The abrasives are much more controllable at low speed. As previously mentioned the battery only lasted ten or fifteen minutes, so I purchased two spares and by keeping them  fully charged at all times I was able to work without interruption.

In practice I found that I used all the diameters, depending on the size and angle of the fillet. The big 60mm was ideal for the wide and shallow chine fillet, which is why I purchased that size.



The final stage was to hand finish the fillets with P80 grit aluminium oxide sandpaper, which I now buy in 50 metre rolls because I use so much of it. That way it works out really inexpensive.

The pipe insulation made quite a good sanding block for the chines but all the other fillets required much narrower blocks. I found that thick cardboard folded into double and triple thicknesses made ideal sanding blocks, offering a solid edge but being slightly flexible they could adapt to the curve of the fillets. A cork block was useful too.



All this sanding took what seemed like an eternity, and I have to tell you that it was mostly abject misery.

Truly, I thought, we have entered the vale of tears.

Other builders told me to keep going, and that once the fillets and fibreglass are completed the build gets back to being fun again.

So I kept the faith and soldiered on until I had sanded the very last fillet. This whole process took two weeks.

Theoretically I should have been ecstatic, marking the occasion with a celebratory glass of something cold, but alas I was only too well aware of an uncomfortable truth … I was not happy with the quality of some of my fillets.

The bottom fillets in the stern compartment were not big enough, and needed to be beefed up quite a lot. They were the first ones I made, and I was over optimistic about how much epoxy would be required.

The same applied to the fillets in what I would normally call the stem of the boat, at the very front. Not big enough or strong enough to take serious punishment if something hit the bow.

And all the keelson fillets were not good enough. They were too thin and patchy, and quite frankly not neat enough!

And here and there around the boat were incomplete or scruffy bits of fillet, which needed remedial work.

There was no doubt that I had a lot more work to do to make the fillets fit for purpose, and that we were still very much stuck in this vale of tears …



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