Fish Eyes in Water-Based Finish

Round craters where the topcoat pulls back from the surface. Almost always silicone or oil contamination. Here's how to find the source and how to recover the floor.

What fish eyes look like

Round craters in the freshly applied topcoat, usually 5-15mm across. The coating runs cleanly up to the spot then pulls back, leaving a ring of slightly thicker finish around a central depression. They show up while the coating is still wet but get worse as it dries.

What's actually happening

Silicone, oil and wax all have lower surface tension than water-based polyurethane. When the coating tries to lay down over a contaminated spot, the adhesive forces in the contamination are stronger than the cohesive forces in the coating, so the coating retreats. Same physics as why water beads up on a waxed car bonnet.

Where the contamination usually comes from

Investigate before you blame the coating:

Furniture polish

If the previous occupant used Pledge, Mr Sheen, or a silicone-based polish, the silicone has soaked into the timber. Sanding doesn't fully remove it -- it sits in the grain. This is the most common single cause.

Body lotion / sunscreen

Bare feet on a previously finished floor leave residue. Sunscreen is brutal. If the floor was a holiday rental or beach house, expect contamination.

WD-40 from the edger

Lubricants used on the edger or belt sander can drip or aerosolise onto the floor mid-sand. Don't lubricate machines on the floor. Don't let WD-40 cans sit on a sanded floor.

Fabric softener / floor cleaners

Some commercial cleaners leave silicone residues. If the client mopped the floor before you sanded, ask what they used.

Other trades

Painters using silicone-based caulks, plumbers using thread sealant, anyone applying RP7 to a hinge in the next room. Silicone aerosolises and travels.

How to recover

Stop coating immediately. The fix depends on when you spot it:

Wet coating:

Wipe back with a clean damp cloth, dry the floor, sand back to bare timber in the affected area, dust off, prime with a sealing primer like Bona Classic UX, then recoat the whole floor.

Cured coating:

Sand back to bare timber. There's no chemical fix -- the contamination is in the timber itself. Some installers spray a shellac sealer over the contaminated spot before re-priming because shellac blocks silicone bleed-through, but on a full floor it's faster to sand and start over.

Prevention checklist

Before you start spreading finish: vacuum and tack-rag the whole floor, ban other trades from the room, ban WD-40 from the work zone, ask the client what cleaners they've used, and inspect raking-light for any glossy or oil-stained spots before priming.

Related: Troubleshooting FAQs →