Here is how I masked for a set of three perfectly straight lines of Brass Gold metallic down the fender. See the pictures at the bottom.1. Find a center line and apply fine line down the middle.
2. Apply fine line tape to either side of that line.
3. Apply another fine line tape line to either side of the two outside lines.
4. Apply green auto masking tape.
5. Remove the three lines to reveal the stripe areas to be painted.
6. Run your fingers over the tape, pressing firmly to make sure it is stuck down firmly and that there are no places for paint to seep under the tape!
7. Use degreaser to prepare before painting, check for dog hairs and lint.
8. Apply a light coat of paint. Allow to flash. Apply a second light coat.
Allow the paint to dry at least two hours and then carefully remove the tape. Do not wait overnight, because that creates a risk that paint will peel when you remove the tape. You should have nice clean lines! Top it off with a clear coat!
TIP: Get all the tape you need for this job in our Paint Project Kit . It has with everything you need (3 types of tape, a mask, degreaser, gloves, sand paper, shop towels) that is discounted (currently by 50%!!) if you buy any paint product.
BONUS TIPS: FIXING OOPSIES.
1. Blurred lines. What happens if the tape lifts and you get a little blur instead of a crisp line? Let the paint dry, tape again, re-shoot that area. Darker colors over lighter colors are going to be more opaque, so that is the preferable order for a touch up. For example, on these fenders, I would lay tape over the brass gold lines (after they are dried) and then I would shoot a light touch up with the black paint to fix the blurs. Some vinyl tapes can stretch or pucker, leaving a gap for paint to find its way under the tape - especially over curved surfaces or when making round corners. If that was the problem, then switch to green auto masking tape made out of paper which will adhere more closely. Any tape residue can be cleaned with degreaser.
2. Runs or build-up lines. You might have a drip from painting too close or too heavily. Or you might have an especially harsh line after you remove the masking tape. Here is our preferred way to fix those: if you are going to clear coat as a final step, then you can wet sand the blemish areas. Focusing on just the problem areas, very lightly sand with fine 800 or 1000 grit sandpaper and water (wet sanding). You just need enough to knock down the unwanted lines. Clean the residue off with degreaser. You may see a little scratchy whitish look where you sanded - that is ok. The clear coat will magically make this disappear and you will only see the color (assuming you cleaned the residue).
More info: this Australian guy at "customspraymods" has a great YT channel with info that you can use in your project.