Using MDF Letters in a Concrete Casting Form

Assuming the wood/MDF letters are mounted to a smooth melamine, phenolic, fiberglass, or steel form to create recessed lettering in the finished concrete, the most reliable professional method is:

Best method: fasten from the back of the form

Attach each letter with short screws driven through the back of the form into the MDF letter.

This is the strongest option for architectural precast work because it resists vibration, concrete pressure, and moisture without allowing the letters to creep or lift. Professional mold makers commonly secure models to melamine baseboards with screws from underneath; epoxy, hot glue, and caulk are secondary alternatives.

Recommended procedure

1. Print a full-size, mirror-reading placement pattern.
2. Position the letters on the clean form surface.
3. Drill small pilot holes through the form from the back.
4. Use short pan-head or flat-head screws that penetrate approximately ¼–⅜ inch into the MDF, without approaching the exposed letter face.
5. Use two screws on larger letters so they cannot rotate.
6. Countersink the screw heads on the back of the form where necessary.

Because the screw holes are concealed underneath the letters, they do not appear in the concrete.

Best adhesive-only method: full-face transfer adhesive

For small and medium letters, especially on one-time forms, use a very thin industrial adhesive-transfer film covering most or all of the back of each letter.

A product such as 3M 467MP, 468MP, or an equivalent thin transfer adhesive creates almost no stand-off between the letter and the form. The 467MP is approximately 2 mil thick; the slightly thicker 468MP provides better wet-out on mildly irregular surfaces. Both are designed for high shear strength and moisture/solvent resistance.

For MDF, I would generally favor the slightly thicker adhesive or a transfer adhesive specifically rated for wood and coated surfaces.

Important: attach the letters to a clean, unreleased form. Apply the form-release agent after all letters are securely mounted. Release agent underneath the adhesive will substantially weaken the bond.

Epoxy: strongest permanent adhesive option

Small dots or a thin film of fast-setting two-part epoxy can secure letters to melamine or another rigid base. Professional mold makers use fast-setting epoxy to secure masters where floating or movement would ruin the mold.

However, epoxy has disadvantages:

• It will damage the form during removal.
• It makes repositioning almost impossible.
• Excess adhesive can squeeze beyond the letter edge and appear in the casting.
• MDF letters will usually be sacrificial.

Keep the epoxy at least ⅛ inch inside the letter perimeter and use the smallest amount that produces a continuous, stable attachment.

Hot glue: acceptable, but not the first choice

Hot-melt glue is commonly used in moldmaking, but it is better for temporary positioning, sealing mold-box joints, or large forgiving shapes than for precision lettering. Polytek Co. identifies hot glue and caulk as workable alternatives for securing and sealing models.

Problems with hot glue include:

• Glue beads can hold the letter slightly above the form. This causes extra post-casting and letterform clean up and finishing.
• Cement paste can seep underneath.
• Letter depth can become inconsistent.
• The glue may soften or release during vibration or warm-weather casting.
• Squeeze-out produces unwanted radiused edges.

For precision letters, use only tiny flattened dots, not thick beads.

MDF preparation is critical

Raw MDF should never contact wet concrete without being thoroughly sealed. Professional concrete-mold guidance calls for mold materials to be nonporous, waterproof, nonreactive, and dimensionally stable. Wood and MDF are porous and must be sealed before release agent is applied.

We recommend:

• Moisture-resistant MDF where available.
• Two or three coats of catalyzed lacquer, solvent-based polyurethane, polyester sanding sealer, or thin epoxy.
• Extra attention to CNC-cut edges and inside counters such as A, B, D, O, P and R.
• Light sanding between coats.
• A final smooth, hard finish with no visible MDF fibers.

Avoid relying on one thin coat. The machined edges absorb far more moisture than the factory faces.

Add draft to the letters

For ½-inch-thick MDF letters, specify approximately 2–5 degrees of inward draft, meaning the exposed face of the MDF letter is slightly smaller than the face attached to the form.

This makes the form pull away more easily and reduces:

• Chipped concrete edges
• Broken serifs
• Letters becoming locked into the casting
• MDF separating from the form during stripping

Straight-sided letters can work for shallow, bold lettering, but draft is much safer for deep letters, small counters, narrow strokes, and serif typefaces.

Should the edges be caulked?

Only when necessary.

If the letters sit completely flat with full-face transfer adhesive, an edge fillet usually is not required. If there is a visible gap, use an extremely small amount of:

• Oil-based, sulfur-free plasticine clay
• Moldmaking wax
• Compatible removable caulk

Moldmaking professionals use clay, hot glue, or caulk to prevent material from leaking underneath a model.

Any edge sealant will create a radius around the finished letter, so the customer should decide whether they want:

• Sharp architectural edges: no visible fillet.
• Slightly rounded, chip-resistant edges: tiny consistent fillet.

Apply release agent last

After the letters are attached and the joints inspected:

1. Clean away dust.
2. Apply a concrete-compatible release agent over the entire form and all letter surfaces.
3. Brush or wipe out excess so it does not pool in corners or counters.

Professional moldmaking instructions recommend even release coverage followed by brushing out the excess to maintain detail.

Heavy release-agent buildup can soften detail and leave staining or finishing problems.

The recommended hierarchy

For commercial precast with vibration:
Back screws plus completely sealed MDF.

For one-time panels and smaller letters:
Completely sealed MDF plus full-face industrial transfer adhesive.

For permanent sacrificial forms:
Thin-film epoxy.

For temporary or noncritical work:
Minimal hot glue.

Avoid ordinary foam mounting tape, thick carpet tape, construction adhesive, and large silicone beads. These hold the letters away from the form and commonly produce cement fins, uneven depth, or rounded outlines.

One final distinction: positive MDF letters attached to the form produce recessed letters in the concrete. To produce raised concrete letters, the form needs letter-shaped negative cavities rather than attached positive letters.