Hot glue fins

Submitted by Phill on 6/23/01. ( )

I know someone is going to say check the archives, I have and maybe I have missed this but I would like to know how to make hot glue fins from start to finish. I am mailing from Australia and what you guys call something, eg. silicone and chaulk are probably totally different products to what we use. I have tried casting the fins in plaster of Paris, waited for them to dry, poured the glue in but I cannot get them to release from the plaster - have tried wax as a release but it seems to interfere with the hot glue. I have been trying to find information and products here, but as the industry here is nothing like you have it is difficult; Please help! Phill

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This response submitted by Bill Gaither on 6/23/01. ( )

I do not personally use the hot glue process to make fins, since there is a recurring problem with air bubbles trapped in the matrix.

Most waxes melt or turn fluid at temperatures below that of the plastic hot glue sticks. The best plaster to use for the molds is the new "dry" plasters from US gypsum company. The plaster is much more dense than other types. It was developed for rapid production figurine casting and quick turnaround time from casting to painting. I believe the water required is around sixty percent less than regular casting plaster, hence a more rapid drying time. The product, called "Dry Stone" has a dry compressive strength of 16,000 (psi) which is greater than hydrocal or hydrostone. Its impact strength is 2,800 g*cm, making it one of the toughest casting plasters to come down the pike in a long time.

The plaster can be sealed either with a high temperature wax with a melting point higher than that of the melt sticks, or by thinning high temperature paint, such as bbq grill paint or engine block paint thinned and sprayed onto the plaster after it has dried. The molds should be cured after painting at a temperature greater than that of the hot glue melt point and then a release agent such as Formula five or a dry silicone release material. The wax should be applied and buffed off after drying.

I make fins by another approach. I mold the fins with the dry stone by pinning them out in the array desired on 3/4 inch insulation foam that is sold at building supply centers. If a curve or other arrangement is desired, I sculpt the foam to the shape desired and then pin the fin to the foam.

When preparing the fins, cut them free from the fish by incisions through the skin along both sides of the fin and then cut the bases off, leaving from 1/2 inch to 1 1/2 inches of the base, dependent on the size of the subject. Leave all meat attached, trimming the rays even with shears or suitable device. trim the muscle tissue up a bit, but it is not necessary to remove all of it. Keep the fins wet during the processing by storing them in a container of water until you are ready to pin them on the foam. Build a small dam of clay around the fin to contain the plaster. Bristol paper or thin aluminum flashing can also be used for the dam. Keys can be made in the foam for register by either drilling a 1/2 inch hole into the foam with a forstner or bottoming bit, and gluing 1/2 inch beads into the foam with five minute epoxy or by cutting small, square keys with a scapel blade and popping the block out of the foam. Keys need only be 1/4 inch deep. Keys may also be cut into the plaster after the first half is cast.

After the first plaster pour has set, pop it off the foam. the fin should remain set into the plaster. Trim any plaster that has run under the edge of the fin, so that the second side pour will have detail all the way to the tip of each ray, or connective membrane. Shave flakes from ivory soap or other soap that is lanolin and creme and scent free. the best is grandma's old lye soap if it can be had. Dissolve the flakes in warm water until the consistancy of shellac. Using a white, vegetable bristle, soft brush, brush the edges of the first mold sides with the soap solution and allow it to soak in. Repeat the procedure at least three times. After the third of fourth application, wax the plaster with a thin coat of carnuba or fiberglass release wax. Allow the wax to dry and then carefully wipe off excess, being careful not to move the fin edges.

Cast the second side of the mold. When the plaster has set, tap the edges carefully and lightly with a rubber mallet until the seam line is visable. Pop the mold by prying carefully with the tip of a dinner knife or a thin piece of shim stock. DO NOT attempt to release the mold from a single pry point. Work around the mold lifting carefully until both sides are separate...... Discard the fins, or place them in water to be frozen for future use.

Check the two halves of the mold for detail and fill any air holes with a bit of plaster, sculpting back to original contour. If the plaster has been applied correctly, there should be no trapped air. Allow the two halves to completely dry. In the case of the dry stone product, this is just 24 hours. Drying may be accellerated in an oven set at 150-175 degrees F. DO NOT exceed 100 C or 212F or the trapped moisture will turn to steam and cause fissuring of the mold surface. When the mold is dry. Apply the soap solution to the entire mold until a soft sheen appears. Allow to dry for several hours and then coat with fiberglass or carnuba wax. Buff off the excess wax after drying.

To cast fins, coat both sides of the mold surface with a single application of gel coat. Clear gel if prefered. While the gel coat is tacking, mix an amount of marine resin gel, (the reinforced type. Using a wooden tongue depressor or other cheap spatula, apply to the fin bases level with each side of the mold, to ensure that the cavity made by the fin base is filled. Mix a small amount of fiberglass resin and use it to coat a precut piece of thin fiberglass mat, shaped like the particular fin. Saturate the mat material with resin, working out any air bubbles with a disposable brush, the cheaper, the better. Place the saturated piece of mat into the fin mold and clamp both sides together have a Flag or a Foster's while the resin sets up, mate.

The cured fins will be clear, if clear gel coat is used and the fiberglass mat nearly invisible. The edges can be trimmed and filed or sanded to a feather edge. If you desire, a new set of blocks may be made and a rigid co-polymer or RTV mold may be made from your casting to allow production. with care the plaster may produce up to a dozen or so sets, dependent on the detail and nature of the fin from which the mold was made.

There are numerous other methods by which fins may be cast, and dozens of materials available from which to make both fins and casts. This method works well for me, and will for you if you follow the procedure above. Email me with any other questions.


This response submitted by Dave Toms on 6/26/01. ( )

Bill I commend you on taking the time to write out what I feel is one of the best descriptions of fin casting I have seen to date. I know it took some time to do. Thanks



This response submitted by Jim Wilson on 6/27/01. ( )

Appreciate you taking the time to share your expertise in this forum. Never really knew about the method you described in detail. I have tried hot glue fins and well they looked like hot glue fins. I would like to try your method. Thanks again for your time on this forum! - Jim

Thanks Bill

This response submitted by Phill on 6/28/01. ( )

Looks like I am a bit slow to say thanks Bill but better late than never. Great info, will give it a go soon.


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