Mistake 1: Skipping Primer
What happens: Paint doesn't adhere properly, coverage is uneven, old colours bleed through, water stains reappear through the new coat. On new drywall that was never primed, the paint soaks into the paper facing unevenly and the finish looks blotchy regardless of how many coats you apply.
The fix: Use primer on bare drywall, any patches or fills, any surface with staining (use a shellac-based stain block for water stains, nicotine, or severe marking), and whenever you're making a dramatic colour change from dark to light. Don't skip it to save time. Primer is cheap compared to applying three coats of expensive paint trying to cover what one coat of primer would have solved.
Mistake 2: Not Washing Walls Before Painting
What happens: Dust, cooking grease, hand oils, and cleaning product residue on the wall surface prevent paint from forming a proper bond with the substrate. The paint may look fine initially and then start peeling or lifting in sheets within months, especially in kitchens and bathrooms. I've seen this exact scenario dozens of times.
The fix: Clean all surfaces before painting. A mild TSP (trisodium phosphate) solution or equivalent degreaser works well. Rinse thoroughly with clean water after cleaning and allow to dry completely. In kitchens, do this twice — cooking grease is tenacious and you need to be certain the surface is clean.
Mistake 3: Buying Cheap Paint
What happens: Cheap paint has lower pigment loading (worse coverage), lower resin quality (less washable, softer cured film), and less UV resistance. You end up applying more coats to achieve coverage, the paint marks more easily, and it doesn't hold up to cleaning. In the end you spend more on paint and labour than if you'd bought quality paint in the first place.
The fix: Spend the money on quality paint. Benjamin Moore Regal Select, Aura, or Sherwin-Williams Emerald are the products I use. The per-litre cost is real, but you need fewer coats and the finished surface lasts significantly longer. On a full interior repaint, the paint material cost difference between a premium product and a mid-grade is maybe $200–$400. That's trivial compared to redoing the work in two years.
Mistake 4: Wrong Sheen for the Room
What happens: Flat paint in a kitchen gets stained and can't be cleaned. Gloss paint on an imperfect wall makes every bump and irregularity visible. Matte paint in a hallway gets damaged within months from bags, hands, and furniture contact. The finish choice determines how durable and maintainable the surface is — not just how it looks initially.
The fix: Match the sheen to the room's use. Eggshell for most living spaces. Satin for high-traffic areas and kids' rooms. Semi-gloss for kitchens, bathrooms, and all trim. Matte/flat for ceilings and low-traffic rooms where you want a softer look. I cover this in detail in my post on matte vs eggshell vs satin.
Mistake 5: Skipping Tape on Trim
What happens: Without tape, cutting in neatly along trim requires a level of skill that takes years to develop. Without that skill, you get paint on the trim, wavy lines where wall meets trim, and a finish that looks amateur regardless of how good the rest of the job is. Trim is the detail that people notice — messy trim lines are the first thing a buyer or guest sees.
The fix: Use quality painter's tape. Apply it to a clean, dry surface, press the edge firmly to ensure a seal, and remove it within 24 hours while the paint is still slightly tacky. For the smoothest edge, score lightly along the tape line with a utility knife before peeling. Don't use the $3 hardware store blue tape — Frog Tape or equivalent precision tape gives significantly cleaner lines.
Mistake 6: Not Patching Properly Before Painting
What happens: Nail holes and small cracks filled with too much spackle, or filled and not sanded flush, telegraph through paint and look worse than the original damage. Heavy-handed patches leave raised ridges that are visible as soon as light hits the wall at an angle. The opposite problem — thin, insufficient filling — leaves a slight depression that also shows.
The fix: Apply spackle in thin coats (two thin coats beats one thick one), allow to dry completely between coats, and sand flush with the surrounding surface. Finish with a coat of primer over the patch before painting. Patches on walls require patience — rushing this step is where most DIY patch jobs end up visible.
Mistake 7: Painting in Wrong Temperature or Humidity
What happens: Interior paint applied below 10°C dries slowly and forms a weak film. Paint applied over 30°C dries too fast, causing lap marks and uneven coverage. High humidity (above 70–75% indoors) slows cure and can cause adhesion problems. In Toronto, this mostly matters for exterior work, but it also applies indoors — painting in a cold garage or a very humid bathroom in summer can lead to problems.
The fix: Maintain indoor temperatures above 15°C and below 30°C during application and for 24 hours afterward. Use ventilation but not direct airflow that creates a wind-chill effect on fresh paint. For bathrooms, run the exhaust fan before and during painting to reduce humidity. For exterior work, check the forecast and don't start if conditions aren't right.
Mistake 8: One Coat and Calling It Done
What happens: One coat of paint is a tint, not a finish. Coverage is thin, the colour is uneven, old colours show through in areas of thinner application, and the surface isn't durable. It might look okay in the middle of the room on a sunny day. It will not look okay in corners, under lamps, or once the furniture is pushed back against the walls.
The fix: Two full coats minimum, with appropriate dry time between them. On significant colour changes or over stained surfaces, three coats (prime plus two finish coats) is correct. There are no shortcuts here. Two good coats applied properly look dramatically better than three thin coats applied too quickly, and a single coat is not a finished paint job under any circumstances.