Molding Surfboard Fins                     home>tutorials>
Warning: Using power tools and fibreglass is dangerous. Proceed at your own risk.


First: The BIG QUESTION. Wink

Why would I bother to make my own fins, (or more specifically, mold them)?

1) Because in the art of sanding foils, I am a Klutz (big 'k').
2) I've got lots of ideas that I want to experiment with and I need to be able to reproduce fins in an evolutionary manner.
3) My face is assymetrical, my head is assymetrical and I surf assymetrically. -yet FCS fins come with symetrical tabs and no fore aft adjustment.. So choice of tab position on my FCS fins.
4) Satisfaction.
5) Masochism. I like to make a big mess and love the feel of fresh fibrgelass itching the inside of my elbows, my neck, my left chest muscles and the back of my throat.
6) Variations in tip flex..Most of the fcs ready mades I have have the flex of dried cheese.
7) Variations in toe in and cant with the same fcs plugs.
8 ) Reduced waiting time for fins
9) Fin box fins are damn hard to get

As an aside: I once overheard a fin foiler bragging he could foil a fin in five minutes. In comparison, it takes me ten minutes to make the fcs tabs fit in the fcs plugs.
Moral: If you are thinking of saving money by molding your own fins - you will. Just don't plan on paying yourself more than 27 cents an hour. (heh)

Well, before we start, a disclaimer.


 If you're not familiar with fibreglass I'd better say that fibreglass is nasty stuff - so always read the instructions first. Wear safety glasses, suitable breathing gear (the best you can afford), lots of longsleeved clothes, work in a nice well ventilated environment.... And as far as MDF, well MDF fibres will kill you in the end, so don't breath them..

What you need for the mould.

*about 1.5 kilos of plaster of paris.
*scrap wood to build a box big enough to hold four fins.
*extra virgin olive oil (release agent)
*baking paper to cover the base of the mold or alfoil
*16mill mdf, enough to cover base of mold and top of mold
*old paint brushes from the time you decided to do a van gogh reproduction for the kids bedroom
*thin scrap mdf

Firstly, make a wooden box to hold the mould.
Leave the box open at top and bottom.
Make it large enough to contain your fins and leave a few centimteres all around for room to play
Aim to have the sides of the mold more than twice as deep as the thickness of the fins.
Attach wooden tabs to the outside of the mold (see picture). You use these to screw the mold onto the temporary base to stop the plaster leaking.
If you use wooden corner brackets they will give you something to screw the real base into later (see photo) once the mold has set.

Cover the temporary base of your mold (2cm mdf) with baking paper. Or alfoil - whatever you can keep wrinkle free.
Screw the mold to the base temporarily to keep it flush with the base and reduce plaster leakage.

If molding from fcs fins, tape thin mdf on either side of the tabs.

Make this mdf the width of the fin base - this way when you come to casting the mold, you will end up with a slab of cloth to cut the tabs out of - which will be much stronger than tabs made of eg milled glass. NB: cover the mdf well with the electrical tape, because the wet plaster has an affinity for mdf and makes it difficult to remove.

If molding from fin box fins work out where they will sit in the box and drill a small hole to take the "pin" so that the fin sits flush.

Once the box is made and screwed to the temporary base, lay the fins you want to mold flat side down on the base inside the box. (Remember that some fcs fins have an inner concave - plaster will leak around this, but that's okay - anyway, you shouldn't be molding commercial products as they probably have a copyright)

Paint the fins with your chosen release agent (extra virgin olive oil first press), making sure you cover all the fin surface.

Mixing the plaster:

Wear gloves. Read the destructions on the plaster carefully. When you mix it, try not to get air bubbles. Once mixed, carefully pour the plaster around the fins and bring it up to the level of your box. Vibrate the plaster a little - this helps get rid of the air bubbles. Before it dries, scrape the plaster level with the top of the mold with a scrap bit of straight wood.

Drying time depends on how well you read the directions. (I don't read real good - I had to wait two days.) If the top of your mold is not perfectly flat sand it down or scrape it with a chisel till it's flush with the sides of the mold.

While you are waiting, cut out the real mdf base of the mold. This has to be reasonably thick because it will support the mold. When the plaster appears to be dry screw this new base to your mold (the mold is still upside down here) using the corner brackets illustrated in the picture if the walls of the mold aren't thick enough.

Unscrew the old temporary base that was covered in baking paper.

Carefully turn your mold over.
Your fins will be stuck in the mold.
Gently use a chisel or flat screwdriver to pry the fins out. Pry them out from the base of the fins, because if you damage the mold here it is not too bad.

If all goes well you will end up with something that looks like this, but cleaner. NB: This is the top view of a used mold.

The plaster may still be a bit wet. Wait a few days before the next step, as you don't want wet plaster touching you resin. Water and resin don't mix. Hydrophobes and hydrophillics.

Sand off any high points on the mold so when you place the lid on it and weigh it down it will sit flush.
At this point you can cut small grooves in the plaster to bleed the excess resin off during the molding process (I didn't which is a pity)

Glassing/ Casting:

For the next step you need

1) 6 ounce cloth (see note below) or
2) 4 ounze cloth
3) some tissue cloth
4) lam resin and catalyst
5) petroleum jelly as relase agent (yep it has another use!)
6) aluminium foil.
7) some really heavy things to use as weights.
Cool thin MDF to use as the positive plug.
9) a selection of paper clamp thingies. (Butterfly clips?)
10) your wife's really, really good kitchen scissors.
11) electrical tape.
12) the aforementioned mold.
13) the MDF base you previoulsy covered with baking paper (This becomes the lid of your mold)
14) packet of milled glass
15) packet of microspheres - optional
16) safety glasses, breathing mask, lots of disposable gloves
17) 3 or 4 big old icecream containers
1Cool scrap mdf to use as a squeegee.

note re cloth:

a) you want between 25 and 35 layers of 6 ounze cloth in each fin.
b) you need enough tissue cloth paper to cover each fin on both sides - consider this optional.
c) the 4 oz cloth seems to give more flex - not sure about this. You will need more layers (40? 45?)

Step one : on a wide flat area unroll a length of cloth.
Place fin on cloth and fold the cloth horizontally twice so that each fold is the same height as the fin. Cut this peice of cloth from the roll.
Fold it over and over like a map so you have say 10 layers of cloth about the size of the fin.

Use a butterfly clip at top and bottom to hold all the layers together.
Trace around the fin with pencil and cut out the cloth with your wife's good kitchen scissors, making sure you leave cloth around the butterfly clips so they hold the cloth together.

Remove the top butterfly clip. Cut the parts of the cloth that are joined together at the top. Replace clip. Do the same with the bottom clip.

You need three groups. Arguably for the best flex the top layer (flat side of fin) should have more layers than the curved side. (see note at bottom)

Repeat step one twice. 15 layers in one. Ten layers in the next. Five layers in the last (these go into the mold first).

Take some of your offcuts and, holding them together with another butterfly clip, cut these so they are staggered in height to about two thirds of the fin.
These offcuts will fill in the meat of the fin.
Attach the offcuts to the clip that already has ten layers.
This will be your middle wad of cloth when you get to the molding.

From the tissue cloth cut out two pieces of tissue cloth in the shape of the fin. This will become the first layer of the fin and be pressed into the curved shape of the mold and the last layer. It will save you having the weave off the cloth as the outside layer of your fin.

Now that you have cut out the thirty layers (15, 10+ offcuts, 5) have a dry run by pressing the cloth into the mold. Trim off any really wayward peices with your wifes good kitchen scissors.

Bear in mind that you will be compressing the cloth and that once it is wet it will be more compact.

Make the male plug. Trace out your fin shape on thin mdf. Cut it out with whatever comes in handy. Maybe make it about two mills bigger than your fin. You use this to concentrate the weight of your really heavy objects on the fin part of the mold, rather than on the surrounding flat surfaces.

Pic: Slightly used male plug in action:

To get the right thickness for your fcs tabs, place your original fcs fin in the mold.

Pack out under the tabs with mdf off cuts, till the top of the tabs are sitting flush with the top of the mold.
Remove the original fcs fin.
Coat the MDF with petroleum jelly and leave in place in mold

In this pic of a used mold the dark brown thingo is the mdf packer to get the tabs the right thickness

Now: get ready:for the CASTING

Go to the loo. Make yourself a coffee and put it in a safe warm place.

Do the next step in a well ventilated area.

Coat the the mold with petroleum jelly.
Make it liberal but keep it thin.
Get the piece of MDF you are using for the top of the mold ready by covering it with aluminium foil. Tape off the foil on the back with your electrical tape.

Coat the male plug with petroleum jelly (“I like petroleum jelly, petroleum jelly for me”)

Put on safety glasses, Open the disposable gloves and try on a pair of gloves for size (they're not all the same size!). If they fit, lay them aside as a spare cause you never know when you are going to need them, and resin on your skin is GOOD WORD stuff.

Put on another pair of gloves.
Lay out your three lots of cloth..
Turn off the phone.
Arrange your bags of milled cloth or microspheres in a handy place.

With your MDF squeegee, in one icecream container, mix up 100 mills of lam resin with enough catalyst to give a slow setting mix. This gives a nice working time. (EG: It was 11 degrees for my last fin and I used 60 drops, which was for a normal day temperature. Remember you are going to be compressing the resin and it has a self reflexive exothermic reaction - so it will get hotter than normal in a mold.)

NB: I have heard catalyst will blind you

Coat the inside of your mold with a thin coating of resin. Press the single layer of tissue cloth into it with your squeegee making sure you trap no air. If all goes well, this will be the outside of your fin.

Soak the first wad (5 cloth layers) in the resin (Its handy to have two icecream containers so you can pour from one to the other). Keep the butterfly clip on and lay this soaked cloth in the mold. Squeegee the cloth to keep out the air.

Once the top 2/3rds is soaked and pressed into the mold release the clip and wet the area that was under the clip.
Keep squeegeeing cause you want as little resin as possible in the finished product.

Repeat this with the middle bundle of cloth. This has the padding for the meat of the fin. Keep squeegeeing, take your time cause you have plenty of it.

Step back, take a look at the centre part of the fin. If you have cutt off enough packing for the meat of the fin it should be reasonably flat. Most probably it won't be. That's okay. In another icecream container mix up a very stiff paste of either microspheres or milled glass with a small amount of catalysed resin. (Microspheres for less weight, milled glass for strength) You want enough to fill in the depresssion in the meat of the fin..

Bear in mind, this will alter the flex of the fin so keep it away from the tip.

Lay the final layers of cloth (15 layers) and squeeggee out excess resin.
Check for bubbles - there will be plenty- and try and squeegee them out.
You will have lots of excess fibreglass strands everywhere . If they are bothering you cut them off with your wife's good kitchen scissors.

After checking once more for bubbles and squeegeeing out as much resin as possible, mix up a very stiff paste of catslysed resin and milled glass - enough to coat the whole flat face of the fin in a thin layer. Lay it in as flat as possible. This will be filler and give you less chance of getting air bubbles in this layer. Most of it will be squeezed aside in the next step.

Lay the final piece of tissue cloth over the fin. and make sure it is suitably wetted.

Carefully lay the male plug down over the fin shape in the mold.

Place your aluminium foil covered mold top ontop of the male plug.
Gradually weight up the top of the mold with heavy weights, making sure you keep it even and the top of the mold level.

spare tiles always come in handy:

Take off your gloves and throw them away.
Your coffee should be luke warm now.
Go and drink it.

Hang around do something else to the resin starts to gel in the icecream containers.
Go and make yourself another coffee and come back with a stanley knife.
Check the resin mix again and if it is hard like surfboard wax is soft you can start unloading the weights off the mold. The resin in the mold will set slightly quicker than the outside resin.

Take off the top of the mold, remove the male plug and carefully cut around the mold, giving the fin about three mills leeway. If the glass weave starts to craze then wait a little longer or cut further out..

At this point you can decide where you want the fcs tabs and trace around your original with said stanley knife.\

Clean up any excess drips on other parts of the mold.

Leave the fin a few more hours then pry it out of the mold. If the resin is still soft you can cut off the excess with the good kitchen scissors. (it;s a matter of timing this part)

Show the resulting 'almost fin" to your wife. Don't tell her about the scissors and she will tell you how clever you are.

Presto: you have a kind of fin shape surrounded by lots of excess glass (for this one I went for a three hour session while it was setting and missed out on the stanley knife/kitchen scissors window). You can see I didn't use the tissue cloth on this one. The weave is six oz. Much less smooth than 4 oz.

Step Finito:

Wait a day or two. Do a test sand on the edge of fin and make sure the resin has hardened fully.

If it has, put on your sanding gear. Cover up every inch of available skin (think: harem girls in public), cause glass fibres itch like ... well like, fibreglass. Put on your safety spex, and your breathing gear. Grab the black and decker workbench and place it in the middle of the backyard as far from the wife's washing as possible.

You need course, medium and fine sandpaper, your wood rasp (square and curved profile), the electric jig saw, orbital sander, spare bits of wood as sanding blocks.

Cut off all excess hardened cloth around the fin planshape with electric jig saw or files or both. Leave the excess at the base of the fin till last as this gives you something to clamp onto.

Using the coarse sandpaper and the orbital sander, take the rough spots off the flat side of the fin. Try and keep at least two thirds of the sander on the fin surface and this will help you stop rounding off the edges.

Reduce the coarseness of the sandpaper till you think its smooth enough. Keep it flat though.

Turn the fin over. If you've been a good boy (unlike me) you will have a nice smooth surface with no air holes.
If neccessary use a long thin sanding block to sand the back two thirds of the fin foil.

Reduce sandpaper coarseness as necessary and patience requires.

Take care not alter the foil. You want to keep the foil, that's the whole reason for molding the fin. When satisfied, place the fin upright in the clamping device.

Clean up the planshape of the fin with a rough file.

Then sand the leading edge with a piece of sandpaper curved to follow the contours.

Then clean up the back with a piece of hand held sandpaper. Be carefull with the trailing edge of the fin, to keep it nice and curved. Use the sandpaper scrunched up into a ball in the approximate diameter of the back of the fin. Imagine the fin is a knife that is going to hit you at least once in the surf. Take all sharp edges off accordingly.

now you may have something like this Smile

Time to cut out the fin tabs.

Place the original fin over the cast fin.
Trace out the fin tabs with a sharpened pencil.
Very carefully cut them out with the jig saw.

Use a square profile file to clean up the corners.
Make sure the base of the tabs is parralel with the base of the fins.

Check to see if they fit in the fcs plugs. If you are a klutz like moi they will be too wide or slightly too seperated. Check for thickness/thiness.

Reduce tabs as necessary. Do this slowly as this is the crucial part of the process. Try to keep the tabs the same thickness from base to end. If they are thinner on the end they can slip out of the fcs plugs (yes, it happened to me)

When you are happy with the fit, wind the grub screws into the tabs gently.
Undo the grub screws and remove the fins.
You will have small indents where the grub screws hit the tabs.
Check out your sample fcs fin and observe the small “wells” in the tabs.
Select the right size drill bit and drill a shallow well in your own tabs.

Go surfing:

Four 'finished“ fins, complete with air bubbles, but hey, I made them.

Things to do wrong

-if you don't add a 'drain' for the excess resin in the mold you want have control of where it goes
- if you don't use the paste in the second last layer (before the tissue paper and male plug are in place) you will most likely have large air bubbles
- if you are too keen with your sanding you will alter the foil
-don't use too much paste in the meat filler - you will reduce your flex. So make sure you cut out plenty of layered cloth to fill the "meat" of the fin
- make sure you keep 2/3rds of the orbital sander on the flat face at all times
- mix up excess resin rather than too little - the small amount of resin is cheap
-if you don't wear long sleaved shirt while sanding then they will be calling you "itchy' for a few days Smile
-if you don't keep a spare pair of gloves on hand you maybe trying to open the glove bag with resin soaked hands
-if you leave your wife's "good" kitchen scissors coated in resin lying around in an obvious place give me a call and I'll let you have my barrister's name

cheers Idea

hope this makes sense

(regards to fin box fins, I read a good thread on swaylocks - let me know if you want the essence
[b]Addendum re flex[/b[

Note: I could be wrong on these two issues ( I know, it's hard to believe;)):

Regarding flex. My wife advises me that you get the most positive rebound energy from tissues that are in flexion rather than compression. Ie When we turn a board pressure on a fin that flexes means that the fibres on the inside part (flat) of the fin are in flexion, the outside part in compression. It makes more sense to have these fibres that are in flexion to be in the longest more unified group - if we want a more dynamic response.

As far as 4 oz versus 6 oz she also tells me that more numbers of smaller fibres will give more energy rebound than less numbers of larger fibres.

with special thanks to the guys on the spoon thread, red for having the guts to make a spoon,
and to the swaylocks fellows who couldn't quite figure out what release agent really works Wink

next week ...... finishing your fins so they look pretty Rolling Eyes and don't hum when you go real fast