Fins Casting

Hi Everyone, I would like to share with you my first attempt in fin casting… I made the mold with silicone and before casting the resin I kept the mould straight and sealed, clamping 2 pieces of plexiglass with them.The result is not bad. I know  like that the fin wont be much resistant but can be added fibers of glass or carbon to give the necessary strenght.I found that the pigment gives flexibility to the resin, so i thought maybe adding fibers and pigment might give to the fin strenght and flex.And to avoid airbubbles just use a vibrator that dental techniciand use for casting plaster into impressions.Let me know what you think about it and share your experience please.

So, you’re casting solid resin as the first step? What you show in the pics looks pretty damn good. But I have to wonder about strength and brittleness. If I was going to attempt something like this, I might try to sandwich a layer of glass in the center, at the very least. Or, figure a way to add some type of reinforcing element in the initial cast.

I have not tried it but have considered it.  Maybe the cotton flock re-inforcement method used by duuuude for his self-built, epoxy resin fin boxes could work for a resin-core fin with FG skins.

Good stuff Fishbone, good stuff SammyA, Stoneburner, and by extension Duuuude.

Was it easy to work with the rubber mold material? I have yet to try it.



The result looks very good!

Getting rid of the bubble in the leading edge will be very difficult with this mould.

You should make a channel from the tips to the top of your mould so that the air can escape.

Great job! I cannot wait to see a fibre reinforced one.



Ye I was thinking the same Hans but I am going for another technique.I’ll have to add fibers now so I think it would be much better if i mixed up fibers and resin, then fill up the mold, close and press it.The air bubbles should just escape because of the positive pressure inside the mold.I will post some pics

Hmm I’m not sure that the air bubbles will escape the way you describe, they can still be pressed into a corner.

But I guess that I do not understand what you are trying to do, go for it and show us the pictures!

It works when I make dentures.I will see what comes out :)).I just love to try new ways to make things. I will start in few minutes

Can you also share the way you make the molds?


On my hard-sided mold I put a small air vent between the tabs and it helped. It only needs to be 3mm (1/8") or so. The base is problematic due to the sharp angles.

Please tell us more about your mold-making materials and process, it sounds like it is backed with professional experience.

Would an old loudspeaker attached to the mold act like the device you use? What sort of music would work best?!?



This is the stuff I bought to add strenght to the resin.Are chopped fiberglass ropes.By the way my wife is helping me in all the steps and she’s having much fun…she’s stoked!!!

Ok I  did the casting and that’s the result.Still air bubbles on the base of the fin, but nothing that cannot be fixed, everything else looks great.

Then in the next step I tried to trim off all the edge of the fin and do the casting again with a different colour, still with fibers and this is the result…and this time no air bubbles at all! woohooo!!The fin has some flex but it’s not fully cured yet.I’ll have a try even with vynilester resin to try to make the fins more stiff…I’ll sand and polish the fins soon and of course go for a crash test in the water :)) Stay tuned!!

good job so far, maybe try vacuum infushion or at least de-gasing the resin that should help or cure the bubble problem. 

The fin-inside-a-fin thing is awesome, makes one think about how to combine cores and coatings. I want to try this with wood on the inside to reduce weight and maybe allow the fin tabs to break before a surfboard gets damaged.

Yes, could you please show us? Pretty please?

This is a project I’ve always wanted to try.  My only question has always been how a do it yourselfer would create a re-usable mold for the center fin.  Side fins seem easy to create a mold for.  The center fin would need to be a 2 piece mold.  I’ve always wanted to come up with a good use for the wasted cloth and resin that glassing boards creates.

To do a center fin, you press modeling clay, into a shallow tray to retain the silicone, then set a fin into the modeling clay up to the mid point of the fin.  Then pour silicone over the fin/ clay.  Wait for the silicone to set.  Then remove it all, and strip off the modeling clay. 

The next step is to set the silicone mould with the fin still in place into the shallow tray. With the fin exposed, and the first silicone mould on the bottom.   Pour a second application of silicone over the fin/ first mould.  Wait for it to cure. 

Strip it all out.  Now remove the fin and you have two halves of a center fin mould.

thanks everysurfer that was  simple

I didn’t do that way everysurfer…i will post some step by step soon…I am just extremely busy with work…stay tuned :)))

Interesting thread, thanks for sharing!

Any updates on how the fins turned out, and how you made the molds?