Beginners Guide On How To Stop Fibreglass Sticking To A Mould

From automotive and aerospace applications to recreational activities and DIY projects, working with fibreglass is incredibly advantageous in a range of fields of complex manufacturing and simple item moulding jobs. However, even some of the most experienced fabricators can run into an exasperating issue... and that's fibreglass sticking to a mould.

Usually the issue of fibreglass sticking to a mould stems from a fairly small mistake or simple oversight you've made during the preparation and production process. But do bear in mind, once the part has cured and is stuck, it can be a real headache. When fibreglass unintentionally, or accidentally, bonds to a mould, it's an expensive and a very time-consuming problem to resolve as you rush to replace materials and salvage your work from irreparable damage.

You may have felt that dreaded "sticking" feeling already; where instead of a clean release you end up fighting to get your fibreglass part out - sometimes cracking, warping or destroying your work in the process. Even worse, the mould can be damaged permanently, requiring time-intensive repairs or even whole replacements.

For professionals and part-time hobbyists alike, these kinds of sticking setbacks are more than just annoying - they’re very costly.

The good news is that fibreglass sticking to a mould is completely preventable, as long as you follow a few key steps, prepare your work thoroughly beforehand and know which products to use. So whether you’re working with polyester, epoxy or vinyl ester resins, a consistent release system will significantly reduce the risk of unwanted adhesion problems. Essentially, you're looking at cleaning, sealing and protecting your mould surface, so investing a bit more time upfront in mould prep leads to a far smoother production process.

If you manage to successfully prevent fibreglass from sticking, you'll not only protect your mould, you'll also create a smart and dimensionally-accurate finished product that is low in imperfections and high in professional quality. Additionally, clean releases reduce the need for sanding and finishing, giving you all the benefits of an efficient fibreglass moulding process and a consistent end product.

In our latest blog, Fibreglass Supplies will lay out all the solutions you need to stave off sticking and get that easy, smooth release every time. This will save you hours of reworking on your project and will also preserve the life of moulds, tools and any other parts you're using. Whether you’re building a boat hull, car panel or a simple fibreglass feature for your home, a good release is the final reward for your prep work.

Stick to these steps, and your fibreglass parts won’t stick to anything else.

1. Cleanliness Is Key: Start With A Spotless Mould

Before any wax or release agent goes near your mould, ensure the surface is 100% clean. Residues from previous layups, dust or even fingerprints can interfere with the release process.

What To Use:

• CTI MultiSolve Adhesive Remover - removes wax, grease, oil, paraffin, printing ink, adhesives, sealants and silicone. For fast, thorough and safe degreasing and cleaning that leaves no oil film on your chosen surface.
• Pure Acetone (if compatible with the mould surface) - comes with a quick evaporation rate which ensures minimal residue, leaving surfaces clean and streak-free.
• Lint-Free Multi Cloths by Smartora - Perfect for cleaning and preparing fibreglass moulds with a lint-free finish.

Pro Tip: Clean wearing gloves to avoid transferring oils from your hands to the mould surface.

2. Repair & Prepare The Surface

Even minor surface flaws like chips, scratches or pinholes can lead to adhesion problems. These imperfections give resin something to grip, increasing the risk of sticking.

Quick Surface Prep Steps:

• Sand the surface smooth with a fine grit (800–1000 grit for high-gloss finishes). Mirka Autonet Sanding Discs are ideal for professional-grade sanding and are available in all grit options to suit your needs.
• Fill imperfections with a tooling gelcoat or resin-based filler.
• Polish the mould surface to restore gloss (especially on older moulds).

A smooth, glossy mould not only looks professional - it performs better too.

3. Apply Multiple Coats Of Mould Release Wax

This step is your first real barrier between the resin and mould. Choose a high-quality mould release wax and apply 4–6 coats, especially for new or reconditioned moulds.

How To Wax Correctly:

• Apply a thin, even layer of wax, such as Honey Wax Mould Release.
• Let it haze.
• Buff off with a clean cloth.
• Wait 10–15 minutes before applying the next coat.

Pro Tip: Always use a clean cloth for each coat to avoid reapplying removed contaminants.

4. Add A PVA Or Semi-Permanent Release Agent

For added protection, use a release film over the wax layer:

Option A: PVA (Polyvinyl Alcohol)

Water-soluble and excellent for new moulds or complex shapes.

• Apply with a spray gun or soft sponge brush.
• Let it dry to a smooth, even film.

Option B: Semi-Permanent Release Agents

Ideal for production moulds and longer runs.

• Applied with a wipe-on method and allowed to flash off.
• Typically need fewer reapplications than wax or PVA.

Key Point: Never skip a release agent if you’re using polyester or vinyl ester resin as these bond aggressively to moulds.

5. Control Your Work Environment

Even with perfect prep, external conditions can sabotage your work. Fibreglass layups are sensitive to temperature, humidity and contamination.

Control The Workspace:

• Keep ambient temperatures within the resin’s recommended curing range.
• Avoid dusty or dirty environments.
• Store resins and release agents in climate-controlled conditions.