If by patchy you mean a streaky finish, the best way to get it perfect is to apply a mist coat as the top layer. You do this by adding water to the emulsion, Make a must coat by adding one part water to three parts paint, mixing thoroughly, and then applying carefully always roller if in the same direction.
Sandpaper Helps Even Out Paint Streaks, Clumps, and Drips
Pizzazz recommends using between 120 and 150-grain sandpaper to sand down raised, uneven areas, streaks, and splotches. Make sure to sand lightly, or else you could remove the layer of paint you applied.
You could try primer at patch areas first, then paint that areas. If everything is going good, then primer all over and paint two coats.
Try to maintain a wet edge. When using a roller keep the open side towards the area just painted. Try to complete one full surface at a time. Paint in an even grid pattern from top to bottom.
Paint and primer combination products do work in certain situations, but every paint job is different. Paint and primer have two different, distinct functions and many times work best as two separate products. Paint provides color, hiding, sheen, scrub resistance and protection against the elements.
DIY beginners sometimes assume that one coat of wall paint will do – and end up with patchy or thin paintwork. Usually, you'll have to apply at least two coats of paint to achieve a smooth, even finish. Applying multiple coats also helps build up the paint so it's more durable, and less prone to chipping.
The larger patches show through because of differences in the texture of the surrounding wall, which has had several coats of paint applied over the years. So you have roller texture and smooth drywall patches.
Not Fully Loading the Roller
The roller needs to be fully loaded with paint to the core, otherwise you'll get air bubbles on your wall, resulting in “paint acne.” To avoid this mistake, roll back and forth on the roller tray until all bubbles are gone and the roller feels heavy.
Dilute the paint by 5 to 10 percent with water for latex-based paint or mineral spirits for oil-based paint. Stored paints can lose moisture, making it more difficult for touch-ups to match the original coat.
Uneven edges usually happen when painter's tape isn't used, or you paint too close to the edge of a surface. Messy lines are the worst. Avoid this by working in small sections and use a great tape to get a nice, even finish. If the damage is already done, simply go back over the edge.
Without primer, the touched-up spot will stand out, and it'll look a right mess. The trick is to blend the new paint with the original surface. Priming will also prep your surface for paint and seal any stains - especially those that wouldn't come off.
Lightly sand the troublesome area with 150-grit sandpaper, then tack the walls. That's painter lingo for “Wipe it with a cloth to get rid of dust.” (Easier than the lingo makes it sound!) Then re-paint the area, maintaining a wet edge—meaning, don't let the paint dry until you finish the wall.
What might take three or four coats of paint can be accomplished with one coat of primer and maybe two coats of paint. Don't even think about painting wood paneling without a few coats of stain-blocking primer. Without a good primer, it could take upward of six coats of paint to cover the wood grain.
Of the two, satin is more durable and is slightly less likely to scuff or mark than eggshell and it's easier to clean when it does get marked. This makes it a better choice, especially for woodwork such as painting skirting boards, than eggshell for high traffic areas like hallways and living rooms.
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Uneven paint can cause a space to look unfinished, whereas a well-painted smooth ceiling gives a polished feel. Patchy ceilings can also be caused by streaks from the paint roller, brush marks or stipple from an uneven roller.
If your roller runs dry, you'll find yourself with uneven patches, leaving your walls looking inconsistent. Also resist the urge to rework wet paint and trust the process… it will look patchy whilst drying! Another common culprit in achieving a patchy effect is not maintaining a wet edge whist cutting in.
How to fix uneven or blotchy paint: No matter how many additional coats of paint you add, this problem likely requires going back to the drawing board. If you run into uneven or blotchy paint, the best thing to do is start over.