Painting a water line on a boat is a relatively easy undertaking, as long as you properly prepare the surface of the boat for paint. Avoid attempting to paint the bottom of the hull, however, as it is much easier to paint the upper part of the boat. Choose a colour that coordinates with the existing colours of the boat. For instance, use a colour similar to the colours used in the boat manufacturer's logo or the same colour as the carpet inside the boat.
Clean the surface of the hull where you will paint the waterline using wax and grease remover. Apply the chemical to one rag, wipe the surface, and then immediately use a dry rag to wipe away the residue.
Measure down from the bow, or front point of the boat, to determine the depth of the waterline. To paint a proper waterline, you need to know how much water your boat displaces. This information is available in your boat owner's manual or can be found by launching the boat and marking where the boat rides on the water. For a purely decorative waterline, between 12 and 16 inches -- measured down from the bow -- is sufficient.
Measure an equal distance down from the corners of the transom as the distance measured from the bow.
Affix one end of the blue painter's tape to the spot you marked on the transom, then unroll the tape as you walk forward to the bow of the boat. But don't allow the tape to touch the hull.
Look down the length of tape when you have reached the bow of the boat. Pull the tape until it is slightly taut, but not tight enough to pull the tape off of the transom. While watching the tape, slowly move toward the point you measured on the bow. If you're using 1-inch painter's tape, it will be much easier to create a straight line. Allow the tape to gently contact the hull as you move toward the measured point on the bow. When complete, do the same on the opposite side of the boat.
Affix a piece of tape between the points on the transom where you affixed the tape for the sides of the boat. Press the tape into position to prevent paint bleeding through. When this is completed, use the masking tape and masking paper to mask off the upper rails of the boat and the bottom of the hull. Left showing is the part of the boat's hull to be painted.
Scuff the surface of the hull with the grey automotive scuff pad. This will ensure the paint you spray will adhere to the surface, which it cannot if there is any gloss left in the paint.
Spray three coats of marine-grade paint onto the scuffed paint, starting with a light misting tack coat. Follow this with a medium-heavy colour coat, and allow this coat to dry for approximately 10 minutes before applying the third coat. Give the paint approximately two hours to dry, then remove the masking material from the boat. This will give you a crisper, sharper line between the paint and the colour of the hull.
Wet sand the paint after it has been allowed to dry for at least 24 hours, using the 1,500-grit wet/dry sandpaper with water. This will smooth out the surface of the paint and help eliminate the heavy paint line between the paint and the hull.
Buff the paint with an orbital buffer and automotive polishing compound until it becomes shiny and bright. Take care, however, not to break through the paint to the original surface of the hull. To make certain of this, use the slowest setting on the buffer.
Apply a pinstripe of contrasting colour between the paint and the hull to set off the colour you painted.