Mold release and leak-proofing for epoxy pours

A deep pour is only as good as the box it sits in. A form that leaks drains resin you paid for, and a form the epoxy bonds to may never come apart. Both problems are solved before you mix, and resin manufacturers publish consistent advice on how.

What epoxy sticks to, and what it does not

EcoPoxy’s casting-mold tutorial gives a simple list (EcoPoxy):

Epoxy sticks to	Epoxy does not stick to
Wood	Sheathing tape (for example Tuck Tape)
Glass	High-density polyethylene (HDPE)
Aluminum	Polyethylene and polypropylene
	Teflon, nylon, Mylar
	Silicone

Everything in the right-hand column can serve as a mold surface or lining. Everything in the left-hand column needs a release layer between it and the resin.

Choosing a mold board

EcoPoxy names melamine, medium-density fibreboard and smooth plywood as the best substrates for building a mold, clad with mold release tape (EcoPoxy). Gougeon Brothers’ Epoxyworks suggests waxed melamine board for work surfaces, prepared with 2 to 3 coats of automotive paste wax containing carnauba (Epoxyworks).

Release methods

Tape. Both river table tutorials cited on this page line their forms with tape. EcoPoxy recommends covering every mold cavity surface with mold release tape or other sheathing tape, flat and fully sealed to the mold surface (EcoPoxy). Epoxyworks also uses plastic packaging tape as a release where epoxy should stick in one place and not another (Epoxyworks). TotalBoat lists sheathing tape, packing tape or mold release tape for lining custom molds (TotalBoat).

Release agent. TotalBoat recommends a thin coat of mold release on a clean, dry mold to create a barrier between the casting and the mold surface (TotalBoat). Epoxyworks mentions aerosol release sprays intended for thermoset resins (Epoxyworks). Check that the release agent is compatible with your resin and follow its drying time.

Plastic sheeting under the work. Epoxyworks suggests covering the bench with 3 mil or heavier plastic sheeting, from which cured epoxy peels off. It adds a caution: a cup of mixed epoxy that overheats while standing on the sheet can melt it (Epoxyworks).

Sealing seams so nothing leaks

Thin deep-pour resin finds every gap. Manufacturers agree on silicone at the seams:

1. Assemble with screws. EcoPoxy recommends screws so the form is easier to take apart and more likely to be reusable (EcoPoxy).
2. Seal the inside joints. EcoPoxy applies a bead of silicone along all inner joints, smoothed to fill the seams, and lets it set according to the silicone maker’s instructions before pouring. For a first project it suggests an extra fillet of silicone on the outer edges as well (EcoPoxy).
3. Tape and sealant together. TotalBoat’s river table advice is to line the box with plastic tape and run silicone or marine sealant along all seams and joints (TotalBoat).
4. Clean before the pour. EcoPoxy wipes the cavity with a lint-free cloth and denatured alcohol after the silicone has set (EcoPoxy).

Walls, base and level

• Wall height. EcoPoxy recommends walls at least 1/2 in above the top of the pour (EcoPoxy).
• Oversize base. Extending the base beyond the walls lets you see the seams during the pour, in case a leak needs patching, and gives leverage when you take the mold apart (EcoPoxy).
• Level. EcoPoxy levels the mold on the worktable with shims before adding the slabs (EcoPoxy). TotalBoat notes that epoxy is self-leveling and will follow any tilt (TotalBoat), so a tilted form gives a wedge-shaped pour.
• Catch tray. EcoPoxy suggests a polyethylene sheet under the mold to catch any leak (EcoPoxy).

Before you mix

1. Every surface the resin can touch is lined or released.
2. Inside seams are sealed and the sealant has set.
3. Walls are at least 1/2 in above the planned pour.
4. The form is level in both directions.
5. The inside dimensions are measured for the mold volume or river table calculator.