Making a plaster mold of a portion of my FG Canoe

[Nicholas Scheuer]

I bought my 50 yr-old Dolphin FG canoe exceeding cheap because it had a couple of poorly-done repairs that cried out to be redone by a someone competent in the medium. In order to mold a new section of midship gunwale along with several inches of topside extending down from the gunwale I constructed a form-fitting wooden box on the good side of the canoe and filled it with wall plaster (cheaper than Plaster of Paris). My question is, how long should I let the plaster cure before removing the mold? The canoe surface was coated with auto wax and the maximum plaster thickness is generally 2". After the entire bad repair is sawed off, the mold will be clamped over the gap and new fiberglass/epoxy laid in place.

[Todd Bradshaw]

Re: Making a plaster mold of a portion of my FG Canoe

Back in the dark ages when I was a sculpture major in college, we would remove the molds from the original (clay) about a day after the plaster had set up (white molding plaster is what we used) and then we would let it continue to harden for a few days before adding the mold release (p.v.a. or wax) and casting the resin, or in the case of laminates first adding a coat or two of gel coat or epoxy, followed by the cloth layers and more resin. We used to get the polyester gel coat or resin from a local boat company for $5 per gallon, so that will tell you how current this information is, but I doubt the process has changed much. As I remember, the only times we needed the molds to be absolutely bone dry was when we were going to pour molten metal (bronze or aluminum) into investment (a mixture of plaster, sand and vermiculite) where moisture in the plaster might cause vary bad things to happen when hit with the hot metal.

[Nicholas Scheuer]

Re: Making a plaster mold of a portion of my FG Canoe

Forgot the timing since I last made a plaster mold for fiberglass boat parts 40 yrs ago. Plaster thickness and mass was less than this time. Don't want to leave a section of mold surface behind if I remove it from the canoe too early due to damp plaster. It's outdoors and I have it covered to keep it dry from dew and rain.

[sp_clark]

Re: Making a plaster mold of a portion of my FG Canoe

Where'd you source that 'wall plaster'?

Or are you perhaps using 'plaster' as a generic term for the stuff sold in plastic buckets for finishing drywall?

Keep in mind that the surface you most want to remain intact will be the last to dry given its position against the fiberglass hull formwork.

Depending on humidity and thickness, I'd allow maybe five days for that cast to be tough enough to pull away cleanly.

[the_gr8t_waldo]

Re: Making a plaster mold of a portion of my FG Canoe

dry wall spackling dries quite slowly. plaster of paris cures rather quickly.... once hardened it can be lifted freee of the hull and continue to dry in free air..speeding up the whole process

[Nicholas Scheuer]

Re: Making a plaster mold of a portion of my FG Canoe

5 or 10 lb bag at Menard's, not spackling. It's grey, not white like Plaster Of Paris or spackling. No rain last night, and the mold was closely covered by a small tarp, yet there was a pool of water on top of the set plaster this morning, telling me water was wicking up out of the mold all night. Repeating that for several nights should indicate when there is no more water in the mold.

[jpatrick]

Re: Making a plaster mold of a portion of my FG Canoe

I don't think it makes any difference whatsoever how long you leave it. This is because I think the wall plaster will shrink beyond usefulness. You need to use a proper casting or molding plaster.

Jeff

[Nicholas Scheuer]

Re: Making a plaster mold of a portion of my FG Canoe

It's a fairly simple shape.

[rregge]

Re: Making a plaster mold of a portion of my FG Canoe

Not sure about the cost and I know I am late to the party, could you use RTV (Room Temperature Vulcanizing) Rubber?

[JimConlin]

Re: Making a plaster mold of a portion of my FG Canoe

I’d just use fiberglass and epoxy. You’ll need the same materials for the repair.
No mold box required. Finished surface off the bat.
Use several coats of a real paste wax when making the mold and when using it.

[Nicholas Scheuer]

Re: Making a plaster mold of a portion of my FG Canoe

^^^ Not a bad idea at all. Never occurred to me.

[Rod Tait]

Re: Making a plaster mold of a portion of my FG Canoe

Mold release and two part casting resin?

[P.I. Stazzer-Newt]

Re: Making a plaster mold of a portion of my FG Canoe

Make sure any wax is silicone free...

PS Todd's PVA is poly vinyl alcohol a water soluble release agent, Not poly vinyl acetate, which is a glue.

[sp_clark]

Re: Making a plaster mold of a portion of my FG Canoe

^^^ Not a bad idea at all. Never occurred to me.

I figured'd you'd dismissed that option already, maybe for cost involved for a one-off mold? Done all the time for 'industrial' production but takes some material to properly stiffen then hold shape once mold is removed from the 'plug' form.

And yes, auto paste wax (carnauba) or the Minwax furniture wax inna can, NO silicone whatsoever essential. I've used the Minwax stuff slightly diluted with paint thinner many times for release agenting on small stuff. Even beeswax works well.

[Nicholas Scheuer]

Re: Making a plaster mold of a portion of my FG Canoe

^^^ Got plenty of resin (some of it very old) and more cloth than I'll use for quite awhile. Went with plaster cause that's what I used years ago. Removed my wall plaster mold from the hull this afternoon and it is fine. I should'a realized that molds made of fiberglass reinforced by structural wood members is what boatbuilders use for new fiberglass hulls.