Lacquer, whole rooms, rough stock, and heavy paint.
Lacquer and strong solvents dissolve it — the hard limit; natural bristle or spray instead.
Past a single broad face — a wall, a floor, a run of doors, an elevation of siding — the 4″ foam drag-dries and can't hold a wet edge, and no wider foam exists to fix that: it is roller, pad, or fitted-brush work, and this is the width where that handoff happens.
Rough, gritty, or splintery surfaces tear the wedge; heavy-bodied coatings overwhelm it; sharp edges and hardware shred it.
And foam lays finish on rather than working it in, so exterior stain into weathered grain is a bristle stain brush's job, not this one's.
the 4″ lays the longest wet edge in the family and is the least forgiving of dawdling — flow, join laps into wet, and don't stop mid-field. Lay, absolutely never scrub: bubbles are worst at this width; tip off once and walk away. Know the ceiling: if the brush is dragging dry before you reach the far edge, the field is bigger than foam should do — switch to a roller or fitted brush rather than reloading endlessly. Cut edges with the point; clean in water or mild spirits, never lacquer thinner; retire a slumped or torn wedge — a big clear coat magnifies every flaw a tired wedge leaves.
Asked before buying
BUYER QUESTIONS, ANSWERED BEFORE YOU ORDER
What's the 4″ foam best at?
A mark-free clear finish across a wide flat — a tabletop, a flush door, wide shelving — in the fewest passes of any foam brush (§2a).
Why not a wider foam brush?
Four inches is about the ceiling — wider, foam drag-dries and can't hold a wet edge; the next step is a roller or fitted brush (§1, §5).
Difference from the 3″?
The 4″ suits broad open flats; the 3″ suits furniture parts and lays off with less wet-edge pressure (§5).
Can it do a whole wall or run of doors?
No — that's past the format's ceiling; use a roller or fitted brush (§4).
Lacquer?
Never — it melts the foam (§4).
Foam or chip at 4″?
Foam for a broad mark-free finish in poly/latex; the 4″ chip for stripper, glue, oil, and hot solvents that destroy foam (§5).
