Making a plaster mold of a portion of my FG Canoe

[StevenBauer]

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

Plaster doesn’t dry. It sets, like concrete. It’s a chemical reaction, not a dehydration process.

[sp_clark]

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

Plaster doesn’t dry. It sets, like concrete. It’s a chemical reaction, not a dehydration process.

Same with epoxy.

But with 'plaster' and concrete (vs. cement, which is the binder in concrete) there's latent moisture trapped within the material that has to make its way out until the entire part reaches equilibrium with ambient humidity, just like wood sawn green then air-dried. Concrete cures to maximum hardness if kept damp during its cure period, sometimes as long as two or three weeks.

To a certain degree this can be accelerated by raising temp / reducing humidity but it still take time. Todd's comment about exploding casting investment is a good example if by chance some small fragments of plaster somehow make their way into an investment that's otherwise ready for a pour.

[ShorelineJohn]

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

I would suggest You coat the mold surface with a couple coats of resin, I am assuming it's polyester? Wet sanding it may be rough but plaster can be really porous and even with waxing the plaster directly the resin or gel coat will wick into it. You can just let it wick up but then the mold will not pop off cleanly, then it might even be dry sanding/grinding, wet sanding, another coat of gel coat and then a lot more wet sanding.

[robm]

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

Concrete cures to maximum hardness if kept damp during its cure period, sometimes as long as two or three weeks.

Actually, concrete can continue to gain strength for decades after being poured. Although keeping it wet speeds the reaction.

[Nicholas Scheuer]

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

I will be spray painting the mold with auto body primer which serves to close the pores and give me a slightly tougher surface to lightly sand before applying a couple of applications of Turtle wax. The FG will be epoxy, not polyester. I'll give the plaster 10 days to dry out before painting.

[Nicholas Scheuer]

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

Plaster mold worked well for laying up new fiberglass on the really bad side of the canoe (approx 1 sq ft area), however, some of the painted and waxed surface broke loose when taking the mold off the cured FR Epoxy, so it won't work for the repair of the opposite gunwale. Will use Jim Conlin's suggestion of a FRP mold for the smaller gunwale repair.

I've rigged posts on the canoe trailer so that the hull rests on a pair of rope slings, permitting the hull to be easily rolled in order to position either side up, with the bottom perpendicular to the ground. The exterior mold (either plaster or FRP) is made on the "upside", while laminating the patch is done with the hull rolled the opposite way

[NickN]

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

it might be overkill for this application, but Epoxacoat is a useful mold surface epoxy. Easier to work with in this instance (thicker) than regular epoxy and should give a smoother and better-releasing mold surface. Small quantity can be purchased. https://www.reynoldsam.com/product/epoxacoat/

[Nicholas Scheuer]

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

^^^ Interesting stuff. Had not known of it.

[JimConlin]

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

plaster mold……..some of the painted and waxed surface broke loose…

more waxing!!

[Nicholas Scheuer]

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

The new starboard-side gunwale and topside patch has turned out very well. It's not finished, but the new laminations are fair with the original hull and should turn out as well as hoped. Meanwhile, the fiberglass mold for the port-side repair turned out great (should'a done that for the starboard mold instead of plaster) and am confident the patch (not quite as large as the first one) will turn out well, too.
I have made a set of new ash thwarts to replace the original badly weathered thwarts.

[sp_clark]

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

...should'a done that for the starboard mold instead of plaster....

Life is for learning after all, glad you're pleased with the results of your efforts!