When painting, obviously it's the finished product that gets the 'oohs and aahs.' What a lot of people fail to take into consideration is the ground work that it took to get to that point. Priming and painting is critical, yes, but arguably the most important part of a painting project is surface preparation.
Your finished work is only as good as the canvas you start with. Even the highest quality of paint is going to have a hard time sticking to a surface covered in dirt and grime. A finished job will always look amateur if the surface being painted is filled with crevices, gouges, and other imperfections.
The good thing is that much of the surface preparation is basic cleaning. There are instances when wall repair will be needed and there are some differences in the processes between interior and exterior surface preparation. This guide will break down those methods and differences whether painting with a spray gun or roller.
Interior Painting Surface Preparation
Whether you are painting on the inside of your home or the outside, the most basic and essential aspect of paint surface preparation is to clean. Sometimes you can paint over a dirty surface and it will look alright at first, but don't expect it to last. Paint needs a clean surface to cling to for longevity.
Cleaning an Interior Paint Surface
Unlike cleaning an exterior wall which will need pressure to remove years of dirt and grime, the process of cleaning an interior surface is much less rigorous. Most interior walls simply need to be wiped down with a towel and have cobwebs vacuumed from corners.
A bathroom or kitchen wall where there is moisture and mildew present is a bit different however. A solution of 3 parts detergent to 1 parts water (or a chlorine based bleach) is recommended to adequately remove any mold residue. You should also use detergent where grease or cigarette smoke has built up.
Repairing an Interior Paint Surface
Most interior surfaces that get painted consist of drywall, which may take on some gouges over time. 8 ounce containers of Spackle can be purchased for under $10 to repair holes under 4” in diameter. You then simply apply the compound with a putty knife, let it dry, sand it smooth, then apply primer to the area so paint will stick.
Larger holes will likely need to have new drywall patched in and the subsequent seams plastered with joint compound and sanded. It may take a couple applications and sanding to get the area completely smooth. Latex caulk can also be used for gaps near door trim, baseboard, windows, etc.
Removing Wallpaper / Scraping Old Paint
Is that 1970s wallpaper not considered chic anymore? Old wallpaper will nearly always need to be removed before painting over the surface. A wallpaper scoring tool used in a circular pattern helps perforate the old product. Then wallpaper removal spray is used to get the surface wet before scraping it off.
(Note: wallpaper can be painted over but only if it is not flaking and is very smooth. A coat of primer is always needed before painting over wallpaper.)
Old paint that has chipped, flaked, or otherwise become loose also needs to be scraped away. Any rough edges should be sanded down and primer applied to bare areas.
Do I Need to Apply Primer?
If this is a remodel where new drywall has been installed it's absolutely important to coat with primer before painting. Priming the wall gives it a seal and also provides a surface so that the paint can absorb better across the entire wall. Failing to apply primer can cause the new paint to look uneven in color.
If you are painting over a flat paint and choosing paint that is very similar in color, primer usually isn't needed. If the color change is drastic (dark to light) or if the surface has some gloss then primer is better for the overall quality of the job. Instead of two coats of paint, you may be able to get away with one primer / one paint for example. Also if the existing paint is oil-based, primer will help the new latex paint adhere better.
(Note: to tell if a paint is oil-based, dip a cotton swab in denatured alcohol. If the paint does NOT remove it is oil-based).
Exterior Painting Surface Preparation
A lot of the same principles exist between getting an interior surface prepped and preparing an exterior surface for painting. Clean, Repair, Prepare, and Paint – they just take some different methods and a few various painting tools and accessories to get to that point.
Cleaning an Exterior Painting Surface
Exterior surfaces are exposed to hundreds of more elements than an interior wall. Therefore the cleaning process of an exterior wall is much more rigorous. Dirt, grime, and mildew become embedded into the paint surface and need to be removed.
Pressure washers are used for exterior wall preparation not just for cleaning, but also the removal of chalking and peeling paint. Chalking happens when the old paint is broken down by UV rays and moisture and will impede the attachment of the new coat.
It's very important to be safe with respiratory protection if pressure washing a home with lead based paint (in fact professionals with proper PPE should be used to remove this harmful material). You also need to keep the PSI on the pressure washer around 600 for softer woods and should never exceed 1200 PSI. It goes without saying watch out for windows when using a pressure washer as well.
If you don't have access to a pressure washer or don't feel comfortable using one, manual cleaning is still very much feasible. A mixture of Tri-sodium phosphate (TSP) and water creates a great all-purpose cleaning solution that doesn't leave residue behind. Mildewcide may be needed to kill off mold in some areas as well, especially where trees are in very close proximity. You'll also want a scrub brush that's in pretty good shape (and a pole extension) when cleaning manually.
(Note: some people prefer manually scrubbing their home. They feel a pressure washer too close risks damage but too far away doesn't remove all the dirt and grime.)
Repairing Exterior Surfaces
Before you apply one drop of paint to the exterior of your home, now is the time to check for leaks and any other damage to the substrate. Holes and gaps in your home not only leave it vulnerable to water damage, but have a negative impact on your energy efficiency as well.
You may need wood putty to fill in some holes that have developed on the exterior of your home (topped with primer when it dries). It should be noted that you don't need to be nearly as picky on how to paint minor imperfections in outside walls compared to inside. Siding (wood and vinyl) has a natural texture anyways to provide allure and curb appeal.
Prepare Exterior Surfaces for Painting
One thing that does need to be done before painting an exterior wall is scraping away (with the grain) all failing paint. Bubbles, blisters, chips, and flakes need to be removed or else the new coating will not adhere as well and the overall quality of the project will suffer. If the wood starts to peel up with the paint in spots, stop and use a bonding primer instead.
You'll also want to feather (sanding) the edges of the areas you've scraped to create a uniform smooth surface. Another thing you'll have to do is caulk any gaps that may have been created due to components expanding and contracting in the change of seasons. Syliconized acrylic-latex caulk provides the best combination of paintability, flexibility, adhesion, and being waterproof.
Paint Surface Preparation Summary
All of these steps for getting a surface ready to paint may seem extraneous, but in the big picture they actually make the job easier as a whole. You don't want to waste time coming back to paint over areas that look tacky or where paint hasn't stuck. Expert painters almost unanimously agree that any job that starts to fail in a year or so is because steps were skipped in the preparation process. Don't do that.