Fisheye marks — those small, crater-like circles or dimples that appear in a freshly applied polyurethane coat — are almost always caused by surface contamination. The good news is they are fixable, but the fix requires patience, the right preparation, and understanding what caused the problem in the first place.
What Causes Fisheyes in Polyurethane?
Fisheyes form when the polyurethane coating can't bond evenly to the surface beneath it. The coating is repelled in small spots, leaving circular depressions. The most common culprits are:
- Silicone — from furniture polish, spray lubricants (like WD-40), or cleaning products
- Oil or grease — from skin contact, cooking residue, or oily cleaning products
- Wax — from old floor wax or wax-based polishes
- Incompatible previous coatings — especially oil-based products under water-based polyurethane
- Dust or fine particles — settling on a wet coat
- Contaminated applicator — a roller or brush that has residue on it
Silicone is the worst offender because it is extremely difficult to remove completely and even a tiny amount can ruin a coat.
How to Fix Fisheyes Already in a Dried Coat
Once the polyurethane has dried, you cannot simply apply another coat over the top — the fisheyes will reappear or worsen. Here's the correct process:
Step 1 — Let the Coat Fully Cure
Allow the affected coat to dry completely before doing anything. Trying to fix wet polyurethane will make things worse.
Step 2 — Sand Back the Affected Area
Using a fine-grit abrasive (typically 120–180 grit screen or sandpaper), lightly sand the entire affected coat — not just the fisheye spots. You want to scuff the whole surface evenly so the next coat has something to bond to. For a full floor, a buffer with a sanding screen is ideal. For small areas, hand-sanding with a sanding block works fine.
Step 3 — Identify and Remove the Contamination
This is the critical step. If you don't remove the source of contamination, the fisheyes will return.
- For silicone or wax: Clean the surface thoroughly with a solvent-based cleaner or mineral turpentine on a clean, lint-free cloth. Change cloths frequently — you're lifting contamination, not spreading it.
- For oil or grease: Use a degreaser appropriate for timber floors, following the manufacturer's instructions.
- For unknown contamination: A thorough clean with mineral turpentine, followed by a wipe-down with a tack cloth, is a reasonable starting point.
If the contamination is widespread (for example, the floor was previously waxed), you may need to sand back to bare timber and start again. This is where hiring a professional floor sander pays off — they have the equipment to do this efficiently and correctly.
Step 4 — Vacuum and Tack Cloth
Vacuum the floor thoroughly, then wipe down with a tack cloth to pick up any remaining dust. Do not use a damp mop before recoating — moisture can cause adhesion problems.
Step 5 — Recoat Carefully
Apply a fresh coat of polyurethane using a clean applicator — never reuse a roller or brush that may have contamination on it. Work in thin, even coats. Avoid applying in direct sunlight or when the room is dusty.
Choosing the Right Polyurethane
NZ homeowners have access to quality floor polyurethane products from suppliers including Resene, Wattyl, and Dulux. Always check whether you're using a water-based or oil-based product, and make sure all coats — including any previous ones — are compatible. Mixing water-based and oil-based products is a common cause of adhesion failure and fisheyes.
You can find these products at Bunnings, Mitre 10, or PlaceMakers.
How to Prevent Fisheyes in Future Coats
| Prevention Step | Why It Matters |
|---|---|
| Clean the floor with the correct cleaner before coating | Removes oils, wax, and silicone |
| Use a fresh, clean applicator for every coat | Avoids cross-contamination |
| Keep the room dust-free while coating dries | Prevents particles settling in wet coat |
| Avoid silicone-based products near the floor | Silicone is nearly impossible to fully remove |
| Check product compatibility before mixing brands | Prevents adhesion failure |
| Sand lightly between coats | Gives each coat a mechanical key to bond to |
Working Safely
Polyurethane products — especially oil-based ones — contain volatile organic compounds (VOCs). Always work in a well-ventilated space, wear an appropriate respirator (not just a dust mask), and keep ignition sources away from oil-based products. Check the product's Safety Data Sheet (SDS) for specific guidance. WorkSafe NZ has general guidance on working safely with solvents and coatings.
When to Call a Professional
If fisheyes are appearing across a large area of the floor, or if you suspect the floor has been previously waxed or treated with silicone-based products, it's worth calling in a qualified floor sanding professional. They have commercial-grade equipment to sand back to bare timber efficiently, and the experience to identify contamination sources before recoating.
A professional is also the right call if:
- The floor has multiple old coats that need full removal
- You're unsure what previous products were used on the floor
- The fisheyes keep returning despite your best efforts
You can find qualified local floor sanders on tradedirectory.co.nz.
