A straight paint boundary involves more than aligning tape. Surface condition, edge contact and paint application also affect the result.
Prepare the edge
Remove dust and loose material using a suitable method. Align tape without overstretching it and press the paint-facing edge evenly onto the surface. Test porous or delicate surfaces in an inconspicuous area first.
Paint and removal
Follow paint application and drying instructions; excessive build-up can contribute to bleed. Remove tape slowly while observing the surface. Determine removal timing from the tape and paint manufacturers' instructions.