Isolate & cut.

Isolate lives on the rail right after Pick. It doesn't paint — it controls what you can see and where your strokes are allowed to land, so the tools that do paint can reach a surface another part of the model was blocking.

It has two jobs. Cut chops the model along a line you drag and hides everything in front of that line, toward you; the hidden geometry doesn't just disappear from view, it goes un-clickable, so a stroke can't land on the thing you were trying to see past. Guard is the opposite idea: it drops a flat sheet through the model, and every paint tool stops at the sheet — a fill stays on the side you clicked instead of bleeding across to the other.

Nothing is deleted. A Show all pill sits at the top of the viewport the moment a cut hides anything — one tap restores the whole model, paint and all.

• Painting into a crevice. Between fingers, into a cockpit, behind a cape, down the throat of a dragon — anywhere a near surface keeps stealing your clicks.
• Reaching the inside of a closed form. Cut the front off a helmet or a hull and paint the interior wall like it was facing you the whole time.
• Keeping a fill on one side of a join. Set a Guard sheet at the wrist, the neck, or the seam between two parts, then Magic Fill or Brush without the colour jumping across.
• Checking a slice of the model without committing to anything — cut, look, Show all.

Cut — slice along a line:

1. Pick Isolate on the rail, then choose Cut horizontally or Cut vertically. A red line drops across the view.
2. Drag the line to position the cut. Everything in front of it — toward you — hides.
3. Wrong side gone? Hit Flip side (top of the viewport) to swap which half stays.
4. Paint the revealed surface. The hidden side is un-clickable, so every stroke lands on the geometry behind the cut.
5. Tap Show all (the pill at the top of the viewport) when you're done. The model comes back whole.

Guard — stop every paint tool at a sheet:

1. Turn on Guard — the toggle in the Isolate panel, the Guard button floating in the viewport, or press G.
2. Set the orientation — Horizontal, or Vert X / Vert Y for an upright sheet.
3. Slide Position to where you want the line — the wrist, the neck, the seam between two parts.
4. Paint with any tool — Magic Fill, Brush, Mark, Grow. It stops at the sheet, so you only affect the side you click on.
5. Click the other side to work it separately. Slide the sheet away or switch Guard off (G again) to paint across the whole model.

• Cut, then Magic Fill. Once the front is hidden, an interior wall behaves like any other exposed surface — tap it with Magic Fill and it floods to its seams.
• Guard a seam before a flood. Magic Fill can jump a join you can't see. Drop a Guard sheet on it first and the flood stops dead at the line.
• Orbit while cut. The hidden geometry stays hidden as you turn the model, so you can work a recess from a few angles without it filling back in.
• Drag, don't tap, for a cut. The cut follows the line you drag across the view — a stray click won't place it.