Monday, 22 March 2010

DIY - Fins - Making a Mould - Step 1

There are many reasons why you may want to mould your own fins - you may want to save money, to make a shorter copy of a favourite fin or improve the performance of a fin in certain conditions. In my case I want to experiment with Twist, Flex & Fin Rake. By making a durable mould many fins (10+) can be made with little or no damage to the mould itself. If you are wanting to make 1-3 fins, there is a quicker and cheaper method I will cover in DIY – Fins – Making a fin Mould – Quick Mould. 

If you have a range of fins from the same family use the longest to make the mould from.  I am making the mould using a Select 39cm SL7.  These are great fins and 39 is the biggest you can buy.

Materials and Equipment

You will need and Epoxy System(resin & hardener) for making fins and this same system can be used to make a durable mould. By adding fillers such as Talc, Carbon Black and Aluminium powder you can make a variety of gelcoats, putties and fillers that can do just about anything related to fins and moulds. I buy 500ml PP(polypropylene) cups from a health food shop to use for mixing the resin and fillers together. They are cheap and can be re-used if not damaged and are the perfect size for the resin quantities required.

To mix the resin and work with the filler you will need to make some spatulas. Power hack saw (& plain hack saw) blades work well as they are thin and stiff and can be ground easily with a variety of fillets on the tip for mixing and filleting. You will need to grind the teeth off of the cutting edge to make cleaning easier.

The powersaw blades are also great for making a blending stick, stick sand paper of different grades to each side using double sided tape and you have a flexible sanding or blending stick.  These are handy for doing board repairs when you have to sand down filler flush with the surface of the board. 

For making the mould you will need some scraps of MDF of melamine coated chipboard.  I prefer the melamine as you dont need to seal the surface and therefore save time when building the mould. MDF will allow a finer edge when cut with a jigsaw. 

Plastic putty or Car Bog is required when setting up the fin on parting plane. To clean-up the putty put some metho in a detergent squeeze bottle and label it!. Another useful tool is a scraper or chiseled end on the filleting tool or spatula to use for shaping and cleaning of partially cured(green) bog.

Here is the first stage of making the fin mould...


  1. Please make stage 2 like to make a fin and this is a great tut...


  2. Great stuff! This is a massively valuable resource that you are documenting and creating here. Thanks heaps for your efforts. I'm playing with some quad fins. Stage 2 Would be super helpful. Do you have plans or timeframe for releasing the next part? Cheers!

  3. Thanks Todd, I ended up selling that fin after I did the first stage so I will have to start another one. I am doing some speed fins soon and will document the full process.

  4. Excellent. Stay in touch.

  5. Hi Tim.
    I've been guessing at your next step. Do you seal up the bottom of the MDF cutout, fill it with resin and then sit the fin in on the fillet tabs that you created?

    What resin or filler have you been using to make the mould itself? Do you have to seal the MDF first or do you find that it doesn't soak anything up?

    A few quick hints would be super helpful about now.

    Cheers mate!

  6. Hi Todd,
    Next step after waxing the fin is to do a full fillet and leave the fin in place. Clean the bog that squeezes through to the top of the jointline plane with a scraper. Re-fill any areas that are low between the jointline face and the fin edge. Wax the full shelf and fin several times buffing between coats. Now you need to make a border for the mould. Leave enough room for some dow pins or locating bosses. Cut out the outline of the mould from another piece of the mould shelf material. The outline can be straight edged rather than an even offset from the fin outline...
    More info soon!

  7. Thanks for the update Tim. Perfect stuff. The first half of the mould came up really well.

    I'm going to do the second half tomorrow night as the forecast looks great for friday and i'm going to be wanting those original fins.

    I can't visualise where you are going with your instructions for the creation of a mould border. What is the purpose of the border going to be?


  8. Success! Very happy with how that went. The moulds are probably not as refined as yours but the finish seems pretty good.

    I haven't yet incorporated any borders that you mentioned above. I might need a bit of a heads up on that.

    Is there any other pointers you could share regarding what is required to finish the moulds? Are channels required the allow excess resin to flow out of the mould when the two halves are clamped together or anything like that?

    Ooh and then comes the interesting part... laying up a new fin.

    The biggest question on my mind is how do you cut and wet out the outer layers of cloth without fraying and messing up the edges. There is bound to be a really good trick for this. I found one site where a guy was making airplane parts. He would sandwich an oversized layup of cloth and resin between a sheet of aluminium foil and a sheet of plastic and then roll and squeegee out the excess resin and air bubbles. At this point he would cut this whole sandwich to the shape he required, peel off the outer plastic, press the cloth into the mould and then remove the aluminium foil backing. I like the idea but he was working with some big layups and i'm not sure that it would work so well with the precise layup of a fin...

    What is the best method you have found for laying up fins?

    How many layers are you looking at per each half of the mould? It was a bit hard to tell exactly but counted roughly 8 layers per half in that select fin you chopped. Sound about right?

    Any other advice?

    I know this is getting well outside the topic of this post. Let me know if it would be better to discuss this someplace else or to email you directly.

    Cheers Tim

  9. Hi Todd, sorry for the delay in response, I have been busy paving...

    The borders are merely to stiffen the flange and allow the layup behind the mould to have a cast backing. The mould should be sanded with fine paper and cut and polished depending on how neat the donor fin was..

    Laying up the fin:
    I use a cardboard outline of the fin to cut the cloth with a "Olfa" rotary cutter on a cutting mat. The rotary cutter cuts a really neat crisp edge. Starting out the simplest method is to simply cut enough layers to fill the mould. If you want a stiff fin do the outer layers in carbon. For glass, pre-fold the the number of layers you need for each half of the mould by measuring the cloth thickness(put 10 layer together, measure with verniers then divide by 10 to get a good ave. thickness)and dividing it by the mould depth. Fold the glass into rectangles so that you can fit the template on there twice in a yin yang orientation. This allows you to cut plenty of layers quickly without wasting too much glass. The layers you counted for the select fin were very thick glass, probably 400gsm, I use 200gsm(about 6 oz) so about twice the layers you counted depending on the resin to glass ratio.

    Wax the mould and polish at least 5 times to prepare the mould surface. Warm the resin and mould to the same temperature ~25 C. You can use a gel coat or clear resin depending on how critical the surface finish needs to be and if you will paint the fin in filler anyway. Carefuly lay the cloth in the mould and wet out(both halves simultaneously) with a brush, continue adding layers but step them away from the tip end gradually. It should look like the profile lines on a g10 fin but in reverse- inside out.

    More info when I get a chance...

  10. Great job!! I can not wait to see the next step