Sanding is a lot of work, but is it really that necessary? In some conditions, yes, it is inescapable. In other cases, it’s optional.
First, the must-do sanding scenario: oil trim, no matter what you intend to paint over the oil, it must be sanded first. Oil-based paint dries to a very hard finish, it’s the main reason for the legacy of durability oil paint enjoys; oil finishes are also renowned for wash-ability – thanks again to the hard finish.
The hardness of the finish is also the fly in the ointment when it comes to repainting over it. That hard finish needs to be roughed up a little bit before anything new will adhere properly to it – even primer!
Be wary of primers that advertise “no-sanding”, it’s an elixir – a promise of labor savings at the cost of nothing really, you were going to prime anyway, right? So why not get the one that sells itself as a labor saver.
The reasons are two: first, sanding creates a mechanical bond; a rough surface for the primer to stick to is better than a smooth glossy one. Imagine pouring a thimble of paint on your cars’ windshield, and another thimble on your driveway; which one is easier to remove?
By sanding, creating a mechanical bond, you’ve given the primer a roughened surface – a profile – to stick to. On top of the primer, anything will stick very well, without sanding it.
Latex paint or acrylic trim is different than oil in many ways. The most important is sand-ability. These finishes don’t sand nearly as easy as oil, but it’s still important to prepare the surface.
De-gloss shiny acrylic trim paints with steel wool and then clean by vacuum, tack cloth, and then with a non-soapy cleanser such as Dirtex or TSP substitute.
Now, your trim (oil-based, or latex) is ready for the first coat. You can prime if need be; a good scenario for priming is if there’s a lot of staining or heavy prep, or bare wood.
If the existing paint film is largely intact, and now properly prepped, priming is optional.
Primers are designed to accomplish either or both: block stains, adhere to surface. So, if your surface is properly prepped, and in good shape, you’re free to skip primer entirely and paint, unless you’re changing material types.
If you’re going over prepped oil with latex trim, you should prime. If you’re going over prepped latex with oil trim, you should prime.
If you are unsure, prime.
Primers should be sanded the following day, then vacuumed and wiped down before the first topcoat is applied. The 2nd topcoat can go right over the first as long as it is safely within the recoat window – check manufacturer recommendations for this time.
Don’t want to do any of this? Call a painting professional, or look up the PDCA Find-a-painter service to find the pro nearest you.