I'm in the process of refurbishing my foils and I've got to the tricky bit - painting them and getting a good finish.

I painted the undercoat on with a roller then a brush to get rid of the 'orange-peal' effect left by the roller. I've always painted foils by hanging them up by the pivot hole but they swing around and it's a complete pain. Before I get to the final undercoat and the topcoat I'd like to come up with a better method.

I guess I could paint one side at a time on a flat surface but it'll take twice as long and I'm always worried the paint shrinking might make the foils warp.

Any ideas?

What sort of paint?

I've already got some International Toplac and thinners - has anyone tried to get a professional auto sprayshop to paint with it?

Most boatyards spray yachts these days so it shouldn't be too difficult to find someone who will spray your foils for you.

Do people lacquer there foils after they have been sprayed to protect the finish?

Dennis is right, there are several disadvantages to a matt finish. Dirt sticks to matt finishes much more easily, so they need to be flatted regularly cos it's difficult to get them clean by washing. Skin friction is higher, although the difference is only really noticeable if you compare a matt foil with a perfect polished foil. I'd like to bet that no-one on this forum has any idea of the amount of time and elbow grease you need to put in to end up with a perfect foil. Let me give you an idea of the process

1. Sand with 240 dry to giva a key
2. Fill all dings/scratches with hard car body filler
3. Sand again with 360 - concentrate on the repairs first, use a block
4. Prime with 2-pack high-build primer - at least 2 coats sprayed, 15 minutes between coats
5. leave to harden 24 hours at room temperature - if you don't do this, the primer will sink into the sanding scratches after you put on the top coats
6. Guide coat - a very fine coat of matt black aerosol. Don't cover the foil, you're after a coat that is so thin you can still see the primer. Hold the can about 2ft from the surface, and move it quickly.
7. Wet flat with 400, using a block. Flat the whole foil evenly until ALL the black is gone. For the leading edge, fold the wet/dry over on itself several times to make a stiffish pad, and go easy!
8. Fill any pin-holes you can still see - now you see why you put on the guide-coat! - with primer using a small brush. Leave for 15 minutes.
9. Clean THOROUGHLY with a good quality spirit wipe, then tack-rag the surface to remove any last traces of sandimg dust
10. If you have sanded through the primer in any little spots you will need to spot-prime those
11. Guide-coat again!
12. Wet flat with 800, using a block as before.
13. Wash thoroughly with clean water - at least twice
14. Leave for at least 2 hours in a dry place. Airing cupboard is good!
15. Tack rag and spirit wipe again. Don't leave the spirit wipe to evaporate, take it off with a dry lint-free cloth
16. Spray 3 coats of 2-pack poly, 10 mins minimum between them. Cover the edges and tip first, leave for 10 mins, then cover the surfaces, then spray the 2nd and 3rd coat all in one
17. leave for 24 hours
18. Wet flat as before with 1200 until the entire foil is uniformly matt. This will take some time
19. Wet flat with 1500 or 2000 as before. You will feel the difference as you flat the foil. This is very important, as you won't be able to see the difference easily
20. When the whole foil is free of the 1200 scratches, polish with Farecla G3 until you get a mirror finish.

If you do it right, it should look like glass, literally! Now though, you have the problem of keeping it like that! Buy a new foil bag, cos the one you've got has sand ingrained in it, and make a cotton innner bag that you can wash regularly to keep the grit off! The gain in performance is small, but you might notice an improvement in pointing ability. Is it really worth all that time and effort, when you'd probably gain more from time on the water?

Have fun

Neil

Spent 2 days fairing foils once getting them exactly perfect, well as far as I could judge only for the crew on the first outing to ding the bottom by slamming the foil down in shallow water. Got a more careful crew now and just make sure the foils look smooth and feel smooth because sod's law says the longer you spend finishing something the quicker it will get broken.

Me and the missus had a long conversation about this the other night with my POV being that a rougher surface induces laminar flow and hers being that any laminar flow is killed by skin friction - actually it was more like hydrogen bonds or something but she a scientist and I'm an Contender sailor so I'd switched off long before I understood it.

The outcome was that a rough surface probably didn't help much and possibly absorbed more water anyway, so go with smooth. We breifly discussed polished surfaces by I ruled this out some months ago when I slid off a recently polished centreboard and threw a race with an inversion when I could have just about won it with a quick recovery.

Nothing on the telly then?

Even if your foils are designed to run laminar - and not all are, Laser for instance - they will only do so over a tiny percentage of the surface unless the finish is absolutely perfect, ie polished to a mirror finish. The rough surface induces turbulent flow very near the leading edge, so the lower drag of the laminar flow section is lost. Under turbulent flow, the surface is "scrubbed" by the fluid in the boundary layer, and this is where the higher drag of turbulent flow comes from. It goes without saying that the rougher the surface, the higher the scrubbing forces will be, and therefore the higher the drag.

If you think about it, if you slide off your centreboard, the water will too! Intuitively, it's clear that a highly polished surface gives less drag than a rough one. The real world situation is complex to explain, but we don't need to explain it. Trust your intuition! If you want the absolute maximum performance from your foils, keep them scratch and ding free, highly polished and perfectly clean.

This is all based on my hazy memory of my Physics degree from 20 years ago, but I also dug out Bethwaites book, and he did loads of practical experiments that seem to suggest that the performance gain is much greater than I thought it was, so maybe it's worth going to all the trouble after all.

And in case you're wondering, it was work all this out or watch "I'm a Has-Been".

Neil

Intrigued by discussion on smooth v matt.

Golf balls are covered in hundreds of little indentations which apparently make for faster progress through the air.  Wouldn't this work with water?

From what I can remember, the indentations reduce form drag and also help the ball to spin, which in turn the spin creates lift which makes it's progress through the air more efficent. You can't spin foils on boats, so the theory won't work.

Rough vs smooth is perfectly displayed if you put a surface in a wind tunnel. The flow round the smoother foils is quicker, therefore, the foil has less friction and moves through the water quicker. If you want more lift from a foil, you're better concentrating on the aspect ratio and thickness of the chord of the foil. Surface friction increases drag and slows boats down.

I think the matt finish arguement holds true compared to a gloss paint finish, rather than V's a polished surface. If you run your hand over gloss paint, you can feel it "stick". 1200 grit it and your hand slides much more easily. Polish it, and it is better still, but not by anything like as much.

As for the "you'd do better spending more time sailing" argument, sand and polish the boat and foils after the sun has gone down, at home in the "warmth" (I wish) of the garage. Can't sail then anyway.

Rupert - the difference between the feel of a matt and gloss surface is down to the area of contact with your hand. A sanded surface is made up of valleys and peaks. Your hand only contacts the peaks, so the frictional force will be less. When it's running in a fluid, the fliud is in contact with the entire surface. This is a bit of a "flat earth" argument, all the science is on the side of the highly-polished foil.

Tom, I think you're missing the distinction between "smooth" and "fair". If a surface is perfectly smooth, it will be reflective, and therefore shiny.

Neil

The other theory I have heard is the idea that the matt finish traps a layer of water and keeps it still, and then the water is flowing round itself, not round the foil, so resistance is reduced still further. Never been sure about that one, but one of the America's cup teams put alot of money into the idea a while ago, and 3M came up with a stick on sheet which reduced skin friction. I believe it got banned.

Personally, I polish the boat and rudder as it is easier to clean the dinghy park mud off, which I'm pretty sure is slow...

Redback, it's almost certainly a case of the foil not being completely dry when you painted it. If you primed it first the water may have been absorbed by the primer, and it then breaks down the adhesion between the primer and the top coat.

Solution is to sand the whole foil back to gel-coat if it has it, or to epoxy if not, and then dry it thoroughly and start again. Good luck.

Neil