Anything textured, and anything you need to cover rather than finish.
Put a foam sleeve on a knockdown or orange-peel wall and it paints the high points, skips the valleys, and tears its face on the grit.
Put it on brick, stucco, or block and it dies in one wall.
It also holds too little paint to be a wall tool even on smooth drywall — you would reload endlessly; that is a fiber mini's job (see the Pro 94010, or the woven 94017).
And foam bubbles in fast-drying flat latex if you overwork it, so it is the wrong choice for a quick flat-paint touch-up where a short-nap fiber mini would be faster and more forgiving.
A foam sleeve holds little; a dripping foam roller is an overloaded one that will sag and spatter. Roll it out on the tray screen until it stops leaving a heavy stripe. Move slowly. Speed is what turns foam into a bubble machine, especially in fast latex and waterborne clears. Thin coats, more of them. Foam cannot build film; do not ask it to. Lay off one direction. A final light pass in a single direction aligns the film and pops out working marks. Know when to toss it. Foam is inexpensive and hard to clean out of the closed-cell face; in solvent coatings (oil enamel, varnish, poly), using and discarding a foam mini is often more rational than a mineral-spirits bath — which is exactly why this cover also sells as a two-pack (94005) and a four-pack (94037).
Asked at the counter
BUYER QUESTIONS ANSWERED BEFORE THE CART, NOT AFTER THE RETURN
Can I paint a wall with this?
Only a very small, very smooth one, and you'll reload constantly — foam isn't a wall tool. Use a fiber mini or a 9" cover.
Why is my finish full of tiny bubbles?
You rolled too fast or overworked the wet film. Load light, slow down, one-direction layoff.
Do I wash it or throw it away?
With oil/varnish/poly, throwing it away usually beats the solvent cleanup — that's why it comes in packs. With waterborne finish you can rinse it, but foam never cleans as fully as woven.
What's the double round end for?
So overlapping passes feather instead of leaving a ridge — it lets the 4" cover paint a wider surface seamlessly.
Best paints for it?
Gloss/semi-gloss enamel, varnish, polyurethane, lacquer — the level-out-smooth coatings.
