A crisp, straight waterline makes a hull look finished. A wavy or bleeding one is one of the most noticeable faults on a model ship, because the eye follows that long horizontal line from bow to stern. The good news is that the method is simple and needs no special tools.
This guide works for plastic and wooden hulls, full-hull and RC models alike. On an RC model, ballast the boat so it floats at the same line you paint.
Why Waterlines Go Wrong
Most waterline problems come from three mistakes:
- Using the keel as a reference. Many ships sit deeper at the stern, so a line parallel to the keel ends up in the wrong place.
- Freehand marking. Even careful freehand lines wander.
- Paint bleeding under the tape, especially over planking, rivets or plate seams.
The steps below deal with each of these.
Tools and Materials
- A flat surface (a sheet of glass, a kitchen worktop or MDF)
- Blocks, wedges or a simple cradle to hold the hull
- A height gauge: a sharp pencil or fine marker taped to a block of wood. Use a stack of books or a small adjustable stand to set the height.
- Low-tack masking tape made for models, in a narrow width for curves
- Clear coat or the first hull colour for sealing the tape edge
- A burnishing tool: a cocktail stick, the back of a fingernail or a plastic spatula
Step 1: Find the Waterline on Your Plans
Look at the side elevation on your plans or kit instructions. Our guide on how to read a model ship lines plan explains the reference lines. The load waterline (often marked LWL) is the line the ship floats at when loaded, and it is usually drawn as a straight horizontal line. Measure its height above the base of the keel at the bow and at the stern. If the two measurements differ, the ship has drag, meaning it is designed to float deeper aft.
On plastic kits, some hulls have a moulded line or small marks at the waterline. Check them against the plans.
Step 2: Set the Hull Level to the Waterline
Set the hull upright on your flat surface so the waterline, not the keel, is parallel to the surface:
- Prop the hull in a cradle or between blocks so it can’t rock.
- Shim under the bow or stern until the waterline heights you measured are the same above the surface at both ends.
- Check that the hull is upright side to side. Measure from the surface to the deck edge on both sides, or use a small square against the stem and sternpost.
Spend time on this step. Every later step depends on the hull being set correctly.
Step 3: Mark the Line with a Height Gauge
- Set your pencil gauge to the waterline height above the surface.
- Slide the gauge along the surface so the pencil point traces around the hull, keeping it flat on the surface.
- Do both sides and around the bow and stern without moving the hull.
On plastic hulls, some builders lightly scribe the line with a sharp point fixed to the same gauge. It gives a precise edge to tape against, but it is permanent, so be sure of the height first.
Step 4: Paint the First Colour
Paint the lighter colour first. Lighter colours cover darker ones poorly, while a dark colour easily covers overspray of a light one. Paint the lighter colour slightly past the line so the second colour can overlap it.
On many ships the lower hull is red or a similar anti-fouling colour, with grey or black above, but schemes vary widely. Check references for your subject. Let the paint cure fully before taping. Fresh paint can lift with the tape.
Step 5: Mask the Line
- Run the tape along the pencil line, with its edge on the line and the tape covering the colour you want to protect.
- Lay it in short overlapping lengths around tight curves at the bow and stern. Narrow tape bends far better than wide tape. Flexible masking tape made for curves helps on compound shapes.
- Burnish the edge firmly with a cocktail stick or fingernail so it seals into the surface, especially over plank seams, rivets and plate edges.
- Mask the rest of the protected area with wider tape, paper or plastic film.
Sight along the tape from the bow and stern, and from directly side-on. Fix any wobbles now.
Step 6: Seal the Tape Edge
This step is what gives a clean line. Before painting the second colour, brush or spray a thin coat of either clear coat or the first colour along the tape edge. Any paint that seeps under the tape is then invisible, because it matches what’s underneath, and it fills the gap so the second colour can’t follow. Let it dry.
Step 7: Paint the Second Colour and Remove the Tape
- Apply the second colour in thin coats, working away from the tape edge.
- Remove the tape when the paint is dry to the touch but not fully hardened, if your paint allows. Very thick, fully cured paint can chip along the edge as the tape lifts.
- Pull the tape back on itself at a low angle, slowly, away from the fresh paint.
Painting a Boot Topping Stripe
Many steel ships have a boot topping, a band of a third colour (often black) between the underwater colour and the topsides. To paint one:
- Mark two lines with your gauge, one at the bottom of the stripe and one at the top.
- Paint the boot topping colour across the band and a little beyond.
- Mask both edges with tape, leaving only the band exposed in reverse (protecting the stripe), and paint the hull colours above and below.
Or use a thin strip of tape or a decal stripe for very narrow bands. On many full-size ships the boot topping rises slightly towards the bow and stern, so it looks even from the side when the hull is loaded or trimmed. Check photos of your ship for this.
Fixing Bleed and Wobbles
- Small bleed: touch up with a fine brush and the correct colour, working along the line, not across it.
- Wavy section: re-mask that section from the surviving straight edge on either side and repaint.
- Ridge at the paint edge: sand lightly with very fine paper once fully cured, then recoat with clear.
A clear coat over the whole hull afterwards evens out the sheen across touch-ups. Our guide to model ship finishes covers choosing a sheen. If the hull is getting names, draft marks or load lines, put them on after the waterline is done, over a gloss coat. See our guide to model boat transfers and decals.
Waterlines on Wooden Hulls
On planked hulls, seal and sand the wood smooth first so the tape can seal against it. Open grain and plank seams are where bleed starts. On sailing ships with copper plating, the top edge of the copper often acts as the waterline. It is set using the same height-gauge method before plating starts.
Once the hull is painted, mount the model so the waterline sits level on display. Our guide to making a model ship stand explains how to set pedestals or cradles at different heights to allow for drag.